Cellular communications system with centralized base stations and distributed antenna units

ABSTRACT

A microcellular communications network includes a plurality of base station units and corresponding antenna units. Each includes either conventional transmitters and receivers or all digital transmitter and receiver equipment, and interface circuitry to a mobile telecommunications switching office. The microcell traffic output is applied to a frame generator/multiplexer. The output of the frame generator/multiplexer is applied to a digitally modulated laser and conveyed by fiber to a remote antenna unit, which demultiplexes the microcell traffic signal, applies it to a digital-to-analog converter, and applies it to a power amplifier and a main antenna. The received signals are filtered, digitized, multiplexed together and transmitted over the optical fiber back to the base station. The strongest signal is selected for use. Deployment of an all digital microcellular communications system occurs in two stages. Digital filtering of the digitized RF signal is also provided, and a digital passive handoff system provides for an FFT analysis of all traffic in the cell and passive switching in response thereto.

This is a division of application Ser. No. 08/204,660, filed Mar. 2,1994 U.S. Pat. No. 5,627,879, which is a continuation-in-part of U.S.application Ser. No. 08/183,221, filed Jan. 14, 1994, now abandoned,which is a continuation-in-part of U.S. application Ser. No. 08/068,389,filed May 28, 1993, now abandoned, which is a continuation-in-part ofU.S. application Ser. Nos. 07/946,402, 07/946,964, 07/946,931, and07/946,548, all filed Sep. 17, 1992, of which are now abandoned.

FIELD OF THE INVENTION

This invention relates generally to high capacity mobile communicationssystems, and more particularly to a digital microcellular communicationsystem.

BACKGROUND

A conventional cellular phone system 5 is shown in FIG. 1A. Such systemsare currently in widespread use in the United States. As illustrated inFIG. 1A, system 5 has a fixed number of channel sets distributed amongthe base stations 12, 13 serving a plurality of cells 11, 16 arranged ina predetermined reusable pattern. Typical cell areas range from 1 to 300square miles. The larger cells typically cover rural areas and smallercells cover urban areas. Cell antenna sites utilizing the same channelsets are spaced by a sufficient distance to assure that co-channelinterference is held to an acceptably low level.

A mobile unit 10 in a cell 11 has radio telephone transceiver equipmentwhich communicates with similar equipment in base station sites 12, 13as the unit moves from cell to cell. Each base station 12, 13 relaystelephone signals between mobile units 10 and a mobiletelecommunications switching office (MTSO) 17 by way of communicationlines 18. The lines 18 between a cell site and the MTSO 17, typically T1lines, carry separate voice grade circuits for each radio channelequipped at the cell site, and data circuits for switching and othercontrol functions. The MTSO 17 is also connected through paths 19 to aswitched telephone network 15 including fixed subscriber telephonestations as well as various telephone switching offices.

MTSO 17 in FIG. 1A includes a switching network for establishing callconnections between the public switched telephone network 15 and mobileunits 10 located in cell sites 11, 16, and for switching callconnections from one cell site to another. In addition, the MTSO 17includes a dual access feeder for use in switching a call connectionfrom one cell site to another. Various handoff criteria are known in theart and utilize features such as phase ranging to indicate the distanceof a mobile unit from a receiving cell site, triangulation, and receivedsignal strength to indicate the potential desirability of a handoff.Also included in the MTSO 17 is a central processing unit for processingdata received from the cell sites and supervisory signals obtained fromthe network 15 to control the operation of setting up and taking downcall connections.

A conventional base station 12 is illustrated in FIGURE 1B. A radiocontroller unit 22 provides the interface between the T1 lines from theMTSO and the base station radio equipment. Transmitters 23, one for eachchannel serviced by the base station, are driven by circuit 22, whichsupplies each transmitter with an analog voice signal. Next, the signalsare passed to a separate nonlinear power amplifier for each channel, orthe signals may be combined and applied to a single linear poweramplifier 24 as shown in FIG. 1B. The output of power amplifier 24 isapplied through duplexer 25 to antenna 26, to be broadcast into thecellular area serviced by the base station.

Signals received in antenna 26 are applied through duplexer 25 to filter27. Filter 27 isolates the entire cellular band signal from adjacentbands and applies it to receivers 28, one for each channel. The analogvoice signal outputs of receivers 28 are applied to circuit 22. Basestation 20 may optionally include a diversity antenna 26' andcorresponding diversity filter 27' and a plurality of diversityreceivers 28', one for each associated main receiver 28. Whereimplemented, the outputs of diversity receivers 28' are applied tocircuit 22, which would thus include circuitry for selecting thestrongest signal as between corresponding receivers 28 and 28' usingknown techniques.

In densely populated urban areas, the capacity of a conventional system5 is severely limited by the relatively small number of channelsavailable in each cell 11, 16. Moreover, the coverage of urban cellularphone systems is limited by blockage, attenuation and shadowing of theRF signals by high rises and other structures. This can also be aproblem with respect to suburban office buildings and complexes.

To increase capacity and coverage, a cell area can be subdivided andassigned frequencies reused in closer proximities at lower power levels.Subdivision can be accomplished by dividing the geographic territory ofa cell, or for example by assigning cells to buildings or floors withina building. While such "microcell" systems are a viable solution tocapacity and coverage problems, it can be difficult to find space at areasonable cost to install conventional base station equipment in eachmicrocell, especially in densely populated urban areas. Furthermore,maintaining a large number of base stations spread throughout a denselypopulated urban area can be time consuming and uneconomical.

AT&T has proposed a system to solve the problem of coverage in urbanareas without having to deploy a large number of conventional basestations. The system is shown and described with respect to FIG. 1 ofAT&T's European Patent Application No. 0 391 597, published on Oct. 10,1990. In that system a grid of antennas sites 40 is placed throughoutthe microcellular system. An optical fiber network 42 interconnects theantennas with the base station 44. Optical wavelength carriers areanalog modulated with RF mobile radio channels for transmission throughthe optical fiber network 26 to the antennas sites 22. A detectorcircuit 27 is provided for each antenna site 22 to receive the modulatedcarrier and reconstruct an RF signal to be applied to the antenna sites22, for transmission into the microcell area 21. RF signals received atantenna sites 22 from mobile units are likewise modulated onto a fiberand transmitted back through optical fiber network 26 to base station25. All of the channels transmitted from base station 25 are distributedto all antenna sites 22. Also, all the channels transmitted from thebase station 25 can be received from the mobile units in any microcell21 and transmitted via optical fiber to base station 25.

The above-described AT&T system has certain limitations. The ability toanalog modulate and demodulate light, the limitations imposed by linereflections, and path loss on the fiber all introduce significantdistortion and errors into an analog modulated signal and thereforelimit the dynamic range of the signals which can be effectively carriedvia an analog system, especially in the uplink direction. These factorslimit the distance from the base station to the antenna sites.

Moreover, in AM systems an out-of-band signal is required to transmitcontrol and alarm information to and from the antenna sites, againadding to the expense of the modulation and demodulation equipment.Moreover, provision of other services such as paging systems, personalcommunications networks (PCN's) or mobile data services are not easilyadded to analog AM systems such as that shown in AT&T's Europeanapplication.

Furthermore, the AT&T system teaches the use of dedicated fiber linesinstalled for each remote antenna site. It would be desirable ifpreexisting transmission lines or fiber paths could be utilized so thatinstallation of new fibers could be avoided.

Another approach to increasing coverage is disclosed in U.S. Pat. No.4,932,049 to Lee. The Lee patent describes a "passive handoff" systemwherein a cell is subdivided into several zones, with a directionalantenna oriented to cover each zone. All the antenna's in the cell areserviced by the same set of transmitters and receivers. A zone switch isused to selectively connect the transmitters and receivers to theantenna units. In operation, the antenna best able to service a mobileunit on a given channel is connected to the transmitter/receiver pairassigned to the mobile unit by the MTSO, and the other antennasdisconnected from that transmitter/receiver pair. To control theswitching of transmitters and receivers to the antennas, a scanningreceiver continuously polls the strength of signals received at theantenna units on all active channels in the cell. The zone having thebest receiver signal strength is selected as the active zone for theassociated channel. The system disclosed in the Lee patent thus allowsfor improving communications with mobile units while at the same timereducing interference with other cells by directionalizing and limitingoverall signal strength in a cell.

SUMMARY OF THE INVENTION

The present invention provides improved coverage and increased capacityby assignment of reusable channel sets throughout the microcell system,without the need to deploy independent, conventional base stations ineach microcell area. It also provides good dynamic range over extendeddistances as compared to analog systems such as the AT&T systemdescribed above.

According to one exemplary embodiment of the present invention, there isprovided a microcell system wherein a plurality of commonly locatedmicrocell base station units communicate with a corresponding pluralityof microcell antenna units deployed in respective microcell areas. Eachbase station unit includes conventional RF base station transmitter andreceiver pairs, one for each channel assigned to the microcell.Additional receivers are also provided to receive diversity channels.The RF signal outputs from the transmitters are combined and applied toa broadband analog-to-digital converter. The digitized signal istransmitted over optical fiber to a microcell unit. Each microcell unitreceives a digitized RF signal and reconstructs the analog RF signalusing a digital-to-analog converter. The reconstructed RF signal isapplied to a power amplifier, the output of which is fed to an antennafor broadcast into the microcell area.

The antenna units include both a main and a diversity antenna. Theantennas each independently receive RF signals from the mobile units.The RF signal from the main antenna is filtered through a first set offilters, one for each channel assigned to the microcell, and thecombined filtered main signal applied to an analog-to-digital converter.A second set of filters receives the diversity signal from the diversityantenna. The diversity signal is also applied to an analog-to-digitalconverter. The digitized main signal and diversity signal aremultiplexed and transmitted over the optical fiber back to the microcellbase station. The base station in turn includes a pair ofdigital-to-analog converters which reconstruct the main and diversityanalog RF signals for application to the receivers. The strongest signalis selected for use in accordance with conventional diversitytechnology. Conventional circuitry interfaces the transmitters andreceivers to the MTSO.

Thus, the exemplary embodiment outlined above contemplates that themicrocell base station/antenna unit pairs are arranged to provide areusable pattern of channels (as in conventional cellular technology) inthe microcell system. The microcell base station units do not normallyinclude an antenna, and can be located in a convenient and preferablylow cost location, which may be outside of the microcell systemterritory if desired.

According to another exemplary embodiment, the invention may be deployedto extend the coverage in a conventional cell. In this embodiment, thebase station may include an antenna for transmission and reception ofanalog RF directly from the transmitters and receivers, while at thesame time transmitting and receiving from a microcell antenna unit usingthe digital carrier over a fiber as described with respect to the firstexemplary embodiment.

According to another exemplary embodiment of the invention, thedigitized microcell traffic is carried in a frame format to and from theantenna units. Each frame includes a plurality of bits assigned to carrya sample of the digitized microcell traffic, with other bits employedfor control and monitoring of equipment, error detection and correction,and end-to-end point-to-point voice traffic between the base station andthe antenna unit. Alternate services such as personal communicationsnetwork traffic, paging services and mobile data services may also becarried using the framing format.

According to yet another exemplary embodiment of the invention, thefiber carrier may be replaced with cable or other carrier medium.

According to still anther exemplary embodiment, the invention can bedeployed to distribute a single set of channels to a plurality ofmicrocell areas. In this embodiment, a single base station unit sendsthe same set of digitized channels to a plurality of microcell antennaunits, which in turn return the same set of channel signals to themicrocell base station.

Therefore, the invention eliminates the problems associated with analogAM (or FM) systems, such as that illustrated in the above-mentioned AT&Tapplication, by using a digital transport resulting in better signalquality and for greater range between a base station and a microcellantenna unit. As employed in one exemplary embodiment, the inventiongreatly increases system capacity over existing mobile telephone systemswithout the requirement of deploying conventional base station equipmentin each microcell area, and allows for provision of alternative servicessuch as paging systems, mobile data services or personal communicationnetworks. The present invention also improves the dynamic range of thesignal and extends the distance signals may be reliably transported fromthe base stations to the antenna units. In another exemplary embodiment,the invention provides readily for the transmission of control andmonitoring information to and from the microcell antenna unit.

To provide additional advantages, an exemplary all-digital embodiment ofa microcell system is also provided wherein a plurality of commonlylocated digital microcell base station units communicate with acorresponding plurality of microcell antenna units deployed inrespective microcell areas. According to this all digital embodiment,the base stations are fully digital and synthesize a digital signaldirectly from the T1 carrier received from the MTSO. The digital signalis transmitted over optical fiber to the microcell units. The microcellunits receive the digital signal, and construct an analog RF signalusing a digital-to-analog converter. The RF signal is applied to a poweramplifier, the output of which is fed to an antenna for broadcast intothe microcell area. The antenna units receive RF signals from the mobileunits. The RF signal is filtered through a set of filters, one for eachchannel assigned to the microcell, and the filtered signal applied to ananalog-to-digital converter. The digitized signal is transmitted overthe optical fiber back to the digital microcell base station. The basestation in turn directly synthesizes the digital signal onto the T1carrier back to the MTSO. Conventional circuitry interfaces thetransmitters and receivers to the MTSO. Thus, this exemplary embodimentcontemplates that the microcell base station units are fully digital andeliminate the need for RF equipment at the base station as well as foranalog-to-digital and digital-to-analog converters, thus providing theopportunity to reduce both the cost and volume of equipment required atthe base station site, and to reduce maintenance needs on inherentlyless reliable analog equipment. The digital microcell base station unitscan be located in a convenient and preferably low cost location, whichmay be outside of the microcell system territory if desired.

A method which allows for the rapid deployment of a system of the typeusing analog-type base stations while permitting the easy upgrade ofsuch base stations to all digital technology is also provided. Themethod's first stage calls for deploying a plurality of microcell basestation units as described above, each including conventional RF basestation transmitters and receivers, one for each channel assigned to themicrocell.

In the second stage of deployment, the analog base stations are replacedwith all-digital base stations wherein the base stations are fullydigital and synthesize a digital signal directly from the T1 carrierreceived from the MTSO. The digital signal is transmitted over opticalfiber to the microcell antenna units installed in the first stage ofdeployment. The microcell antenna units receive the digital signal, andconstruct an analog RF signal using a digital-to-analog converter. TheRF signal is applied to a power amplifier, the output of which is fed toan antenna for broadcast into the microcell area. The antenna units alsoreceive RF signals from the mobile units. The RF signal is filteredthrough a set of filters, one for each channel assigned to themicrocell, and the filtered signal applied to an analog- to-digitalconverter. The digitized signal is transmitted over the optical fiberback to the digital microcell base station. The base station in turndirectly synthesizes the digital signal onto the T1 carrier back to theMTSO.

Thus, the exemplary embodiment outlined above contemplates that theantenna units installed in the first stage do not need alteration orreplacement when the analog microcell base station units are replacedwith all digital microcell base stations. The method thus allows thefull benefit of the all-digital base station to be accomplished withoutthe expense of modifying existing installed microcell antenna units.

According to yet another alternate, exemplary embodiment, the digitizedRF signal, carrying either microcell or PCN traffic, is framed fortransmission over a switched telephone network. In this embodiment, alimited number of digitized microcell or PCN channels are groupedtogether, in a standard framing format for transmission using a standardDS-3, OC-1, or other protocol.

In yet another alternate, exemplary embodiment, digitized microcell orPCN RF signals are transmitted over the installed fiber infrastructureof a cable system from the head end to the optical nodes, in anamplified modulated (AM) format.

A still further exemplary embodiment contemplates the transmission ofthe microcell or PCN traffic in digital form over the cable systemfeeder lines, using QAM modulation or other digital modulation formats.

Thus, according to these embodiments, microcell or PCN channels may betransmitted over an established switched network or using establishedcable system infrastructure.

According to still another embodiment of the invention, there isprovided a passive handoff system using digital signal analysis torapidly switch transmitters and receivers among different antenna unitsin different microcell zones of a cell.

According to yet still another embodiment of the invention, there isprovided decimation filters for digitally filtering out a selectednumber of channels from the digital stream output from theanalog-to-digital converter, and multiplexing the selected channels ontoone or more lower speed carriers, such as a T1 line or SONET carrier.

According to yet still another embodiment of the invention, a passiveswitching method is described for use in a cellular phone system havinga plurality of macrocells including a first macrocell, each macrocellsharing a common set of channels, the method comprising the steps ofproviding a plurality of primary and secondary microcell antenna units;dividing the first macrocell into a plurality of primary microcells,wherein the step of dividing includes placing the primary microcellantenna units so as to provide coverage over the first macrocell;providing a plurality of secondary microcell antenna units; placing thesecondary microcell antenna units to provide macrocell coverageoverlapping the primary microcells; at a base station, generating adigitized representation of a telephone signal received from a mobiletelephone switching office, selecting a microcell from said plurality ofprimary and secondary microcells and transmitting the digitizedrepresentation to the microcell antenna unit of the selected microcell;receiving, at the selected microcell, the digitized representation,generating a corresponding RF signal by digital-to-analog conversion,and broadcasting the RF signal in the selected microcell; receiving RFsignals in each of the plurality of primary and secondary microcells forthe set of channels, and converting the RF signals received tocorresponding digitized RF signal representations for transmission backto the base station; at the base receiving the digitized RF signalrepresentations from the primary and secondary microcells; andmonitoring the digitized RF signal representations from each of theprimary and secondary microcells and based on the energy level of eachchannel in each zone, selectively controlling the channels broadcastinto each of the primary and secondary microcells and selectivelychoosing the microcell from the plurality of primary and secondarymicrocell in which a received channel is received so that passiveswitching may be accomplished.

According to yet still another embodiment of the invention, a method ofsectorizing coverage over a particular cellular communications area isdescribed, the method comprising the steps of providing a remote unithaving a plurality of microcell antenna units, including a first and asecond microcell antenna unit, wherein each microcell antenna unitcomprises an antenna configured to cover a particular sector and achannel filter unit used to filter channels assigned to the particularsector; connecting the remote unit to a sectorized base station unit,wherein the step of connecting comprises providing a unique sectorfrequency associated with each antenna unit sector; connecting thesectorized base station unit to a mobile telecommunications switchingoffice; generating, at the sectorized base station unit, a digitizedrepresentation of a telephone signal received from the mobile telephoneswitching office; transmitting the digitized representation to themicrocell antenna unit for a particular sector; receiving, at the firstmicrocell antenna unit, a first RF signal, digitizing the first RFsignal and converting the digitized first RF signal to a first sectorfrequency; receiving, at the second microcell antenna unit, a second RFsignal, digitizing the second RF signal and converting the digitizedsecond RF signal to a second sector frequency; and multiplexing thedigitized first RF signal at the first sector frequency and thedigitized second RF signal at the second sector frequency andtransmitting the multiplexed signal to the sectorized base station.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention and its various features,objects and advantages may be obtained from a consideration of thefollowing detailed description, the appended claims, and the attacheddrawings in which:

FIG. 1A is a functional block diagram of a first prior art mobilecommunications system;

FIG. 1B is a functional block diagram of a prior art base station;

FIG. 1C is a functional block diagram of a prior art microcell mobilecommunications system;

FIG. 2 is a simplified block diagram of an exemplary embodiment of themicrocell communications system of the present invention;

FIG. 3 is a more detailed block diagram of the base station embodimentshown in FIG. 2;

FIG. 4 is a more detailed block diagram of the base station shown inFIG. 3;

FIG. 5 is a more detailed block diagram of the framegenerator/multiplexer 134 shown in FIG. 4;

FIG. 6 is a simplified diagram of the structure of one exemplary dataframe;

FIG. 7 is a diagram of the structure of another exemplary data frame;

FIG. 8 is a functional block diagram of a microcell antenna unitaccording to the exemplary embodiment shown in FIG. 2;

FIG. 9 is a functional block diagram of the demultiplexer 142 andassociated interfaces of FIG. 4;

FIG. 10 is a functional block diagram of an all-digital exemplaryembodiment of the invention;

FIG. 11A is a more detailed block diagram of the system illustrated inFIG. 10;

FIG. 11B is an alternative embodiment of the system illustrated in FIG.11A;

FIG. 11C is yet another alternate embodiment of the system illustratedin FIG. 11A;

FIG. 11D is still another alternative embodiment of the systemillustrated in FIG. 11A;

FIG. 12 is a simplified illustration of an alternate embodiment of themicrocell communication system according to the present invention;

FIG. 13 is a functional block design of the alternate embodiment 106' ofthe system of FIG. 12;

FIG. 14 is another alternate exemplary embodiment of the microcellcommunication system of the present invention;

FIG. 15 illustrates yet another alternate exemplary embodiment of theinvention wherein alternate services, such as personal communicationnetwork (PCN) traffic and paging traffic is multiplexed with cellularsystem traffic;

FIG. 16 is a simplified illustration of a prior art cable televisionsystem infrastructure;

FIG. 17 is a simplified block diagram of an alternate exemplaryembodiment of the invention, wherein cable system infrastructure is usedto transmit digitized RF to and from a microcell location;

FIG. 18 is a block diagram of a base station unit of the exemplaryembodiment of FIG. 17;

FIG. 19 illustrates the head end unit located at the head end of thecable system of the exemplary embodiment of FIG. 17;

FIG. 20 is a more detailed block diagram of the AMmodulator/demodulator, located in the head end of the cable system ofthe exemplary embodiment of FIG. 17;

FIG. 21A is a more detailed block diagram of analog-to-digital converter132, as used throughout the various embodiments in the invention;

FIG. 21B is a more detailed block diagram of digital-to-analog converter144 as used throughout the various embodiments of the invention;

FIG. 22 is an alternate preferred framing structure for the embodimentof FIG. 2 of the present invention;

FIG. 23 is yet another alternate preferred framing structure for theembodiment of FIG. 2 of the present invention;

FIG. 24 is a more detailed block diagram of the microcell remote unit tobe positioned at the optical node in the cable system embodiment of FIG.17;

FIG. 25 is an illustration of the amplitude modulator as used in theembodiment of FIG. 17;

FIG. 26 is a more detailed illustration of the amplitude demodulator, asused in the embodiment of FIG. 17;

FIG. 27A is an illustration of a base station of an alternate exemplaryembodiment of the system illustrated in FIG. 17, wherein the RFmicrocell or PCN signal is digitally modulated;

FIG. 27B is an illustration of an alternate embodiment of the systemillustrated in FIG. 17, wherein the RF microcell or PCN signal isdigitally modulated;

FIG. 28 is a further illustration of the alternate embodiment usingdigital modulation;

FIG. 29 further illustrates the construction of the optical node in thedigital modulation embodiment;

FIG. 30 is an overview diagram of yet another exemplary embodimentwherein digitized microcell or PCN RF traffic is framed and transmittedover a switched telephone network;

FIG. 31A is a more detailed block diagram of the base station units ofthe embodiment of FIG. 30;

FIG. 31B is an alternate exemplary embodiment of the base station unitsof the embodiment of FIG. 30;

FIG. 32A is a more detailed block diagram of the analog-to-digitalconverter and framing circuits of the base station units illustrated inFIG. 31A;

FIG. 32B is a more detailed block diagram of the analog-to-digitalconverter and framing circuits of an alternate exemplary embodiment ofthe base station units illustrated in FIG. 31B;

FIG. 33A is a more detailed block diagram of the remote antenna units ofthe system illustrated in FIG. 30;

FIG. 33B is a more detailed block diagram of an alternate exemplaryembodiment of the remote antenna units of the system illustrated in FIG.30;

FIG. 34 illustrates yet another exemplary embodiment of the inventionwherein digitized RF signals are transmitted over a switched telephonenetwork and a cable system; and

FIG. 35A is an overview functional block diagram of an exemplaryembodiment of a microcell communications system, having passive handoffcapability according to the present invention;

FIG. 35B is a more detailed block diagram of an exemplary base stationunit 114' of the system of 35A according to the present invention;

FIG. 35C is a schematic illustration of the movement of a mobile unitfrom one zone to another;

FIG. 36 shows an exemplary embodiment of digital transmitting/receivingunit 130" of the system of FIG. 35A;

FIG. 37 illustrates an exemplary embodiment of controller 810 of thesystem of FIG. 35A;

FIG. 38 is a simplified block diagram of the operation of controller 810of the system of FIG. 35A;

FIGS. 39A, 39B, 39C and 39D are still other alternate exemplaryembodiments of passive handoff systems with all-digital base stationunits;

FIG. 40 is an alternate embodiment of the system of FIG. 35B;

FIGS. 41A, 41B and 41C are exemplary embodiments of redundant microcellcoverage;

FIG. 42 is a simplified block diagram of an exemplary embodiment of asectorized microcell communications system according to the presentinvention;

FIG. 43 is a more detailed block diagram of the base station embodimentshown in FIG. 42;

FIG. 44 is a more detailed block diagram of the remote unit embodimentshown in FIG. 42;

FIG. 45 is a more detailed block diagram of one example of a channelfilter unit which can be used in the remote unit shown in FIG. 44;

FIG. 46 is an alternate embodiment of the base station embodiment shownin FIG. 42; and

FIG. 47 is an alternate embodiment of the remote unit embodiment shownin FIG. 42.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of exemplary embodiments of theinvention, reference is made to the accompanying drawings which form apart hereof, in which like numerals refer to like elements throughoutthe several views, and which is shown by way of illustration only,specific embodiments in which the invention may be practiced. It is tobe understood that other embodiments may be utilized and structuralchanges may be made without departing from the scope of the presentinvention.

The general configuration of one exemplary embodiment of the presentinvention is shown in FIG. 2. The microcell system includes a pluralityof microcell areas 100. Deployed within each microcell area 100 is amicrocell remote antenna unit 102. Such units may be deployed on theroof of a building or within a building, or on or in other structures.For example, a microcell antenna unit 102 may be deployed on each floorof a building on or adjacent an antenna tower, or along a highwaycorridor.

Remote antenna units 102 are connected through fiber 104 (or optionallyanother high bandwidth carrier) to respective base station units 106.Base station units 106 are interfaced to MTSO 110 over T1 lines 112.MTSO 110 is interfaced with a switched telephone network 120, as in aconventional cellular phone system. Microcell base station units 106 arepreferably located in a single location 114. Such location may be insideor outside of the area serviced by the microcell system, but in anyevent is preferably conveniently located for maintenance purposes.

Referring now to FIG. 3 there is shown a simplified diagram of amicrocell base station 106 according to one exemplary embodiment of thepresent invention. Base station 106 includes conventional transmittersand receivers 23 and 28, respectively, and conventional radio controlleror interface circuitry 22 to the MTSO 110. A digitaltransmitter/receiver unit 130 receives the combined RF signal fromtransmitters 23, digitizes the combined signal and transmits it indigital format over fiber 104A connected to a remote antenna unit 102.Unit 130 also receives a digitized RF signal over fiber 104B from aremote antenna unit 102, reconstructs the corresponding analog RFsignal, and applies it to receivers 28. Accordingly, conventionalequipment may be used on the downstream (MTSO) side of digitaltransmitting/receiving unit 130.

Referring now to FIG. 4, there is shown digital transmitting/receivingunit 130 in greater detail. Unit 130 includes a broadband digitizer 132receiving the combined RF signal from transmitters 23. Digitizer 132provides a digitized microcell traffic stream, consisting of a series ofsamples of the incoming analog RF signal. Frame generator/multiplexer134 frames the digitized microcell traffic data, together with control,voice and error checking data, and applies it to a digitally modulatedlaser 136. The voice data channel, also termed the order wire channel,originates from order wire interface 135, which has an input for ahandset 137 or a two-wire phone line. Order wire interface 135 providesfor two-way point-to-point voice grade communications. Typically ahandset is used at the remote site to connect with a handset at the basesite. Control signals originate from control/alarm circuit 131, whichgenerates control information for the remote antenna unit 102 to monitorerror and alarm information.

The laser signal from digitally modulated laser 136 is applied to fiber104A for transmission to the corresponding remote antenna unit 102.According to one possible embodiment, digitizer 132 preferably providesa 24 bit wide word (parallel structure sample) running at 30.72MegaSamples/second (MSamples/s). The frame generator/multiplexer 134converts the 30.72 MSamples/s word to a single serial bit stream runningat 819.2 MegaBits/second (Mb/s).

The digitizer 132 conditions the broadband RF signal by providingbandpass filtering sufficient to eliminate out of band signals, andsufficient gain adjustment to prevent overloading of theanalog-to-digital converter. The analog-to-digital converter convertsthe conditioned broadband RF signal into a parallel bit stream, eitherby direct sampling at RF, or by sampling following down-conversion tobaseband or to an intermediate frequency band. In the preferredembodiment, the digitizer is obtained from Steinbrecher Corporation ofWoburn, Mass., with sampling performed on a 12.5 MHz wide signaldown-converted to either the first or second Nyquist zone, with 12 bitsampling occurring at a rate of 30.72 MSamples/s.

Unit 130 further includes a digital optical receiver 140. Receiver 140outputs an electronic digital signal, which is applied to demultiplexer142, which extracts the digitized microcell traffic data generated atthe remote antenna unit 102, as will be explained further below.Demultiplexer 142 further extracts alarm (monitoring) and voiceinformation framed with the microcell traffic data. The digitizedmicrocell traffic signal is applied to digital-to-analog converter 144,which reconstructs the analog RF signal, to be applied to receivers 28.

The digital-to-analog converter 144 operates on the microcell trafficparallel bit stream extracted by demultiplexer 142, reconstructing abaseband replica of the broadband RF signal digitized by digitizer 132.The baseband replica is then up-converted to its original radiofrequency by mixing with a local oscillator and filtering to removeimage frequencies. In the preferred embodiment, the digital-to-analogconverter is obtained from Steinbrecher Corporation of Woburn, Mass.,and operates at the preferred sample rate of 30.72 MSamples/s.

Referring now to FIG. 21A, there is illustrated in more detail thebroadband digitizer or analog-to-digital converter circuit 132 in FIG. 4and 170 in FIG. 8. Analog-to-digital converter circuit 132 preferablyincludes a local oscillator 132A, which applies its output to mixer132B, which receives the combined output from the transmitters 23. Mixer132B reduces the high frequency microcell signal (approximately 850 MHzin the case of conventional cellular phone service or approximately 1.8GHz in the case of PCN traffic), to an intermediate (or baseband)frequency of approximately 1 to 15 MHz (such that the 12.5 MHz frequencyfits between these limits) prior to application to analog-to-digitalconverter 132C.

Illustrated in FIG. 21B is the digital-to-analog converter 144 and 164,of FIGS. 4 and 8, respectively, which performs the reverse operation ofanalog-to-digital converters 132 and 170. Digital-to-analog converter144 includes a digital-to-analog converter 144A, which outputs anintermediate frequency signal, which is up-converted with mixer 144B,using the local oscillator 144C. Up-conversion restores the operatingfrequency of the RF to the broadcast frequencies of the cellular or PCNsystems.

Referring now to FIG. 5, there is shown in greater detail the framegenerator/multiplexer circuit 134 according to the exemplary embodimentof the invention shown in FIG. 4. Circuit 134 includes a cyclicredundancy check (CRC) generator 155, which receives microcell trafficdata from digitizer 132 and outputs a CRC code.

According to one exemplary embodiment, framer/multiplexer 154multiplexes the CRC channel, microcell traffic, order wire (voice)channel and control (alarm) channel into the frame structure illustratedin FIG. 6. Each frame includes a 12-bit microcell traffic word, a onebit CRC channel, a one bit control-alarm/order wire channel and a sixbit framing word. The control-alarm and order wire data are multiplexedtogether in a single channel.

FIG. 7 shows an alternate frame structure having 12 bits for the mainantenna channel, 12 bits for 12.5 MHz coverage of alternate service ordiversity channel, a one or two bit CRC channel, 1 bit control-alarmchannel and 6 bit frame word. Other possible framing structures couldinvolve a total of 48 information bits for full band coverage anddiversity capability, or for carrying additional services. It shall beunderstood that the present invention is not limited to these or anyother particular framing format, but rather that any format could beused without departing from the scope of the present invention.

To achieve synchronization with the parallel transfer word, the framesignal shown in FIGS. 6 and 7 runs at 819.2 Mb/s (i.e. 32×25.6×10⁶bits/second=819.2×10⁶ bit/second). (The bit rate and sampling rate for40 MHz/48 bit or other frame structure would change accordingly.)Synchronization is achieved at the receiving demultiplexer 142 (162 inFIG. 8 described below) by searching for the frame pattern. Thirty-twoindividual frames are grouped into a superframe. One of the 32 frameshas a bit sequence different from the other 31 frames. Each frame byteis a balanced code having an equal number of ones and zeros. The framesearch is initiated by the demultiplexer 142 to find consecutivepatterns, followed by a search for the unique bit sequence in one of the32 frames. When the frame and superframe are found by the demultiplexer142 (or 162), valid traffic pattern or data patterns result. Framingmethods of this type are well known in the telecommunications arts, andthose of skill in the art will recognize that various alternate framingmethodologies may also be used. Preferably, frame generator/multiplexer134 includes circuitry for scrambling the outgoing data to provide forthe balanced line code preferred for fiber optic transmission.

Referring now to FIGS. 22 and 23, there are shown the alternatepreferred framing structures of the embodiment of FIG 2. As shown inFIG. 22, the framing structure includes 12 bits of PCN/microcelltraffic, one framing bit, one bit of CRC and an alarm-control/order wirechannel, and four reserve bits. The framing structure in FIG. 23 isidentical, except for 13 bits have been allocated to the PCN/microcelltraffic. Neither of these framing structures is designed to accommodatediversity traffic, however, they could be so expanded. The framingstructures of FIGS. 22 and 23 assumes a 12 bit sampling at 30.72 Mb/s.The basic framing structure is 18 bits, which, when run at 30.72 Mb/s,results in a rate of 552.96 Mb/s serial rate. As shown in FIG. 22, onebit is dedicated to framing. Another bit is multiplexed between CRC,alarm-control, and the order wire function. These two bits achieveframing and multiplexing by virtue of the following sequence:

    ______________________________________                                        Framing Bit    CRC, Etc.                                                      ______________________________________                                        00             Frame 1                                                        01             Frame 2                                                        10             Frame 3                                                        10             Frame 4                                                        1C             Frame 5                                                        1D             Frame 6                                                        ______________________________________                                    

As illustrated above, the framing structure of this embodimentcontemplates that six frames make up a "super frame." The first fourframes of each super frame include the 00, 01, 10, 10 sequence. In thefifth frame, the framing bit is a 1, and the other bit represents onebit of CRC code. In the sixth frame, the framing bit is a 1 and theother bit is an alarm-control/order wire channel bit.

Preferably, the CRC code is 32 bits wide, so that 32 frames must bereceived in order to accumulate the entire CRC code. Accordingly, errorsare checked every 32 words of data. As in the case of the previouslydescribed framing structure, a balanced line code is provided.

Referring now to FIG. 8, there is shown a block diagram of the remoteantenna unit 102, according to the first exemplary embodiment of thepresent invention. A digital optical receiver 160 receives the opticaldigital data stream transmitted from the microcell base station on fiber104A. Receiver 160 converts the optical data stream to a correspondingseries of electrical pulses, which are applied to demultiplexer 162.Demultiplexer 162 extracts the microcell traffic and applies the 12-bit(or 13-bit) samples to digital-to-analog converter 164. Converter 164reconstructs the analog RF signal and applies it to linear poweramplifier 24. Converter 164 is preferably the same as digital-to-analogconverter 144 described and shown above with respect to FIG. 4.Amplifier 24 is connected to the main antenna 26 through a duplexer 25.Accordingly, radio frequency signals originating from transmitters 23 inthe microcell base station are transmitted from main antenna 26.Demultiplexer 162 also extracts control signals for application to acontrol/alarm circuit 161. Order wire data is also extracted and appliedto order wire interface 163 to provide two-way, point-to-point voicegrade communication.

RF signals received at main antenna 26 are passed through duplexer 25 tofilter 27. Power amplifier 24, duplexer 25, main antenna 26 and filter27 are conventional base station components, as are described withreference to FIG. 1B. The output of filter 27 is combined and applied toa broadband analog-to-digital converter 170 (of the same type as 144described above with respect to FIG. 4), which digitizes the analog RFsignal and applies it to a frame generator/multiplexer circuit 172. Theoutput of circuit 172 is applied to digitally modulated laser 174, whichapplies the corresponding optical digital stream to fiber 104B. Framegenerator/multiplexer 172 is of substantially the same design asframer/multiplexer 34. It receives an alarm (or monitoring) signal datastream from control/alarm circuit 161, and an order wire data streamsignal from order wire interface 163.

Optionally, remote antenna unit 102 may include a diversity antennasystem 180. System 180 includes a diversity antenna 26', which appliesits output to filter 27' and in turn to broadband analog-to-digitalconverter 170', which operate in the same manner as main antenna 26,filter 27 and broadband analog-to-digital converter 170, respectively.The output of analog digital converter 170' is applied to circuit 172,which multiplexes the digitized RF signal from the diversity antennainto the data stream applied to fiber 104B. In such a case, the framingscheme includes diversity traffic capacity.

Referring now to FIG. 9, there is shown in greater detail demultiplexercircuit 142 (and correspondingly 162) shown in FIG. 4 and FIG. 8.Circuit 142 (162) includes a demultiplexer 190, which receives thedigital data stream from digital optical receiver 140. Demultiplexer 190extracts the control/alarm channel, order wire channel, CRC channel andmicrocell traffic channel from the digital data stream. Optionally,where the diversity function is provided, the diversity CRC channel anddiversity microcell channel are also extracted. The main CRC channel andmicrocell traffic channel are applied to CRC checking circuit 192, whichprovides an error signal to the control/alarm circuit 131. Circuit 131monitors the error rate of data and alarms occurring at the remoteantenna unit 102. The order wire channel is applied to order wireinterface 163, to provide two-way point-to-point communication.

Where diversity is optionally included, a second CRC checking circuit192' receives the diversity CRC channel and diversity microcell channeland produces an error signal which is applied to control/alarm circuit131.

ALL-DIGITAL EMBODIMENT

Referring now to FIG. 10, there is shown an alternate exemplaryembodiment 200 of the present invention. Alternate embodiment 200includes a remote antenna unit 102 as described with respect to FIG. 8.Remote antenna unit 102 is connected to an all-digital microcell basestation 210 through fibers 104A and 104B. Microcell base station 210 isconnected to an MTSO.

All-digital microcell base station 210 is shown in more detail in FIG.11A. Circuit 210 includes a T1 interface 202, which extracts digitizedvoice channels carried by a T1 line or other carrier from an MTSO andapplies those channels in digital form to digital synthesizer 212.Digital synthesizer 212 replaces transmitters 23 and theanalog-to-digital converter 132 of the embodiment shown in FIG. 4.Digital synthesizer 212 constructs, with digital logic or software, anequivalent to the digitized output of broadband digitizer 132 forapplication to frame generator/multiplexer 214. Synthesis may beaccomplished, for instance, by electronic or software simulation of thegeneration of the analog telephone signal and the modulation of thetransmittal signal therewith. The simulated signal transmitter outputsignal can then be directly represented in digital form that can beprocessed to emulate the output of the A/D converter.

An alternate embodiment of the system of FIG. 11A is shown in FIG. 11B.In the system of FIG. 11B, the synthesizer 212' receives an analog inputfrom radio controller 22, and converts the analog output signals(corresponding to analog telephone signals) from the radio controller 22into a corresponding digitized traffic stream. In this process, forexample, synthesizer 212' can first digitize the individual analog inputsignals, and then process them digitally to produce the digitized signalfor delivery to units 106. On the return path, digital demodulator 224'produces a plurality of analog telephone signals compatible with theinput to the radio controller 22. Multiplexer 214 operates in the samefashion as described with respect to frame generator/multiplexer 134described above with respect to FIGS. 4 and 5. The output of framegenerator multiplexer 214 is applied to digitally modulated laser 216,which outputs the optical data stream on fiber 104A. Digital opticalreceiver 220 receives the optical data stream from fiber 104B andapplies it to demultiplexer 222, which operates in the same fashion asdemultiplexer 142 of FIG. 4. The output of demultiplexer 222 is appliedto digital demodulator or receiver circuit 224, which extracts themicrocell channels and applies them to T1 interface 202 for transmissionto the MTSO.

Yet another alternate embodiment of the all digital base station isshown in FIG. 11C. In FIG. 11C, the frame generation/multiplexer 211' ismodified to apply its output directly to the switched telephone network,in a format compatible with network protocols, for example DS1, DS3 orSONET. The switched network is then used to connect the base stationwith each antenna unit 106. According to this embodiment, the modifiedsynthesizer 212" generates a separate digitized output (for example asshown below with respect to FIG. 32B) for each channel being used (asopposed to all channels in the cellular band), such that only thedigitized form of the channels used for each antenna unit 106 areactually transported thereto, thus greatly reducing the bandwidthrequired for this purpose. Similarly, demultiplexer 222' is configuredto receive the individually packaged digitized channels from theswitched network, and demodulator 224" is modified to receive andextract the individual channels. The embodiment of FIG. 11B can also bemodified in this manner, as is illustrated in FIG. 11D.

Thus, all-digital base station 210 synthesizes the effect of digitizingthe transmitter data stream, providing for an all-digital conversionfrom circuit 202 to the data stream applied to fiber 104A. Thesynthesized signal is received at the remote antenna unit 102, whichconstructs the radio frequency signal, using digital-to-analog converter164, thus eliminating the need for transmitters 23. Similarly, digitaldemodulator or receiver circuit 224 eliminates the need for receivers28, by converting the demultiplexed digitized RF data stream directlyinto digital phone channels for application to circuit 202 and transportto the MTSO.

Yet another exemplary alternative embodiment of the invention is shownin FIG. 12. The alternate embodiment shown in FIG. 12 includes a basestation 106', having an antenna 250 for broadcasting and receiving RFsignals into a cellular area. In addition, 106' includes one or moreremote antenna units 102 used to reach shadowed areas. This embodimentis not for the purpose of extending capacity, but rather to improvecoverage.

Referring to FIG. 13, base station embodiment 106' is shown in moredetail. The configuration of FIG. 13 is the same as FIG. 3, except theRF signals are connected simultaneously to a main cell site antennathrough a duplexer and power amplifier.

Referring now to FIG. 14, there is illustrated an alternate preferredembodiment of base station units 106 and antenna unit 102. According tothis alternate preferred embodiment shown in FIG. 14, there are providedwave division multiplexers 270 at the base station 106 and remoteantenna unit 102. Wave division multiplexers 270 provide that a singleoptical fiber 271 can be used in place of a pair of optical fibers 104Aand 104B, as shown with respect to the exemplary embodiment of FIG. 4.Preferably, the wavelengths of operation for wave division multiplexers270 are 1310 nm±20 nm, and 1550 nm±20 nm.

Yet another alternate exemplary embodiment of the invention is shown inFIG. 15. In FIG. 15, alternate service traffic (personal communicationnetwork (PCN) traffic and/or paging traffic as shown in FIG. 15, forexample) are multiplexed into the digital carrier and conveyed to theremote antenna unit 102 for transmission as part of a broadband signalreconstructed by the digital-to-analog converter. The remote antennaunit is modified to include separate analog-to-digital converter,digital-to-analog converter, filter, duplexer, linear power amplifierand antenna for the alternate service. The optical transceiver and fiberbeing shared with the microcell traffic. PCN transmissions are receivedat the remote antenna unit 102 and conveyed on the digital carrier backto the base station 106. The additional services are carried over thesame fiber simply by adding more bits per frame. Therefore, theembodiment of FIG. 15 can carry the traffic associated with severaldifferent alternate services and cellular traffic simultaneously, withminimal cost over straight cellular traffic. It is contemplated that yetother services can be carried, and the invention is in no way limited tomicrocellular, paging, PCN or mobile data service traffic.

Method of Installing and Upgrading the Microcell System

For ease of implementation of the present all-digital embodiment,two-stage deployment is contemplated. In the first stage, microcell basestation units 106, of the design shown in FIG. 3, are deployed. Theseunits may be readily constructed with conventional transmitter andreceiver technology in the base station unit, and conventional interfacecircuitry to the MTSO. In the second stage, units 106 may be replaced orupgraded to all digital microcell base station units 210, wherein theanalog transmit and receive circuits are eliminated. This upgrade may beaccomplished without changing remote antenna units 102, and thereforemay be done conveniently and expeditiously. This method of installationthus allows the initial units 106 to be constructed readily and atrelatively low cost, and thus providing for rapid deployment, whileallowing for upgrade to more reliable all-digital base station equipmentwithout change to the remote antenna units 102.

Thus, as described above, the present invention provides not onlyimproved coverage, but also for increased capacity by assignment ofreusable channel sets throughout the microcell system, without the needto deploy independent, conventional base stations in each microcellarea. Also, by virtue of digital transmission, it also provides gooddynamic range over extended distances as compared to analog systems.

The exemplary configuration illustrated with respect to base station 106and remote antenna unit 102 provides control/alarm/monitoring andtwo-way point-to-point voice channels to be readily multiplexed on thedigital carrier, providing advantages over analog systems such as thatdisclosed by AT&T. Furthermore, a diversity channel can also bemultiplexed into the data stream to provide the diversity functionwithout the need for additional fiber paths.

The invention also permits ready adaptation to carry alternate servicessuch as PCN, mobile data and paging services together with microcellulartraffic.

Another advantage of the invention is its ready adaptation to alldigital base station technology, wherein microcell traffic data receivedfrom an MTSO in digital form can be digitally converted to a synthesizedstream of data samples for application to the digital-to-analogconverter in the remote antenna unit 102.

It shall be understood that other control or monitoring type channelsbetween the base station and antenna units are also possible, and thatthe invention is not limited to the particular channels illustrated inthe exemplary embodiments.

Transmission of Microcell and PCN Traffic Over Cable System FiberFeeders

A conventional cable system is illustrated in FIG. 16. System 300includes one or more satellite dishes 304 receiving satellite televisionsignals from satellite 302. In addition, the head end may receive videofeeds from local sources or over other media such as fiber, coaxialcable or line of sight microwave link. Video unit 308 provides videosignal splitting, and provides a video signal to AM transmitters 310,which apply an amplitude modulated signal, typically down-convertedprior to transmission, for application to a fiber feeder. The fiberoptic feeder transmits the video signal to a optical node 312, whichprocesses the received signal for delivery to a plurality of homes 314,typically over copper coax cable, or in state of the art installations,over a fiber link. In a typical suburban installation of the type mostadaptable to the benefits of this exemplary embodiment, an optical node312 preferably provides service to approximately 250 homes, covering ageographic area of approximately 1-2 square miles.

An exemplary embodiment of the present invention, wherein the cablesystem 300 is utilized to transmit microcell or PCN traffic to microcellareas will now be explained with reference to FIG. 17. The system ofFIG. 17 provides the advantage of using the installed infrastructure ofa cable television system to transport microcell and PCN traffic. Asshown in FIG. 17, the head end of the cable system includes a head endmicrocell/PCN unit 332, video multiplexer 308, and a plain old telephoneservice (POTS) and data source 336. Although the provision of POTS anddata service is included in this exemplary embodiment, it is notnecessary to the delivery of cellular/PCN service, and may be omittedfrom the system. Preferably, the POTS/data are carried on a plurality ofsubcarriers within a certain band. A separate subcarrier would beassigned to each subscriber in the system. Similarly, the video channelsare also contained on a plurality of subcarriers in a specified band. Inaddition, the microcell/PCN channels are also carried on separatesubcarries in a defined band. The head end unit 332 is interfaced with abase station unit 330, through a pair of fibers 331A and 331B. Basestation unit 330 is interfaced to the switched telephone network 320through a mobile telephone switching office (MTSO) 322.

The head end further includes a plurality of AM modulator/demodulators338, which are coupled to microcell optical nodes 342 through fibers340A and 340B. Optical nodes 342 each include an antenna for thetransmission and reception of microcell or PCN traffic and areinterfaced to a plurality of subscriber homes 343. POTS/data source 336,multiplexer 308 and head end 332 are each connected to the respective AMmodulator/demodulators 338, as more fully illustrated in FIG. 20, to beexplained further below.

Base station unit 330 is shown in more detail in FIG. 18. Unit 330functions identically to unit 106 as described above with reference toFIG. 3. The base station unit 330 may be positioned, as in theembodiment of FIG. 2, in a convenient location, remote from the headend. Alternatively, base station 330 could be located at the head end,with the elimination of the fiber link and other unnecessary components,such that the RF signal output of the transmitters may be filtered andapplied directly to the AM modulator/demodulators 338 and in return theoutput of the AM modulators/demodulators 338 filtered and applieddirectly to the receivers 28. Digital transmitter/receiver 130 of basestation 330 is configured as illustrated in FIG. 4.

As shown in FIG. 19, head end unit 332 is configured substantially thesame as unit 102 from the embodiment of FIG. 2. RF digitization andframing for communication between the head end unit 332 and base station330 is performed substantially the same as described above with regardto units 102 and 106. However, the output of digital-to-analog converter164 is applied to filters 335, which filter the RF signal into aplurality of bands, each to be delivered to a particular microcellassociated with a optical node 342. In the exemplary illustrativeembodiment illustrated herein, the channels of the microcell or PCNsystem are divided into a plurality of 1 MHz bands, each containing aplurality of channels of microcell or PCN traffic (for example ten 100KHz channels or approximately thirty 30 KHz standard channels). Each ofthe AM modulator/demodulators 338 (shown in detail in FIG. 20) receivesa 1 MHz band of channels and conveys it to the microcell optical node342 over the fiber 340A and 340B by AM modulation. In the reverse path,1 MHz bands are received back from the microcell optical nodes 342 (overfiber 340B), demodulated in an AM modulator/demodulator 338, filtered byfilters 337 and combined before application to broadbandanalog-to-digital converter 170, on a return path to the base stationunit 330.

Referring now to FIG. 20, there is illustrated in more detail amodulator/demodulator 338. Each unit 338 includes an AM modulator 338A,which receives a POTS/data input signal, a video input signal, and aPCN/microcell traffic input signal. AM modulator 338A combines thesignal inputs and produces an AM modulated signal for application to AMoptical transmitter 338B, which in turn applies its optical wavelengthoutput to fiber 348. On the return path, AM demodulator 338C receives aninput from AM optical receiver 338D, and provides an output ofPCN/microcell and diversity traffic, together with a POTS/data outputsignal.

Referring now to FIG. 24, the microcell optical node unit 342 is shownin more detail. Unit 342 includes an AM optical receiver 400, whichreceives the AM modulated signal from an AM modulator/demodulator 338.The output of optical receiver 400 is applied to an AM demodulator 402,which outputs a POTS/data signal and a video signal. The POTS/data is tobe delivered to subscriber homes over the optical node to home datatransmission medium. The video signal is also supplied to subscriberhomes over the transmission medium (usually coaxial cable, or possiblyfiber). The PCN/microcell traffic is separately outputted from AMdemodulator 402 and applied to an up-converter comprising mixer 404 andlocal oscillator 406, where it is restored to its operating frequency.The signal is amplified with amplifier 408 and applied through duplexer410 for transmission into the microcell area via a main antenna 412.According to the exemplary embodiment disclosed herein, the channelscarried in the 1 MHz band, are transmitted from the antenna unit at theoptical node. RF signals received at antenna 412 are fed throughduplexer 410 and are applied to filter 420. Diversity antenna 424 mayoptionally be provided with its output applied through filter 426 to amixer 428. A local oscillator provides an input to mixer 422 and mixer428, effecting a down-conversion of the received PCN or microcelltraffic before application to AM modulator 432, together with returnPOTS/data traffic. AM modulator 432 combines the main channel, diversitychannel and POTS/data signals and modulates them onto fiber 340B throughAM optical transmitter 434.

The AM modulators (338A, 432) and demodulators (338C, 402) areillustrated in more detail in FIGS. 25 and 26. Referring now to FIG. 25,there are shown in more detail AM modulator 338A. The POTS/data, videochannel and microcell/PCN channel signal sources are applied torespective mixers 350, 352 and 354, where they are frequency shifted toa desired frequency for combination at combining circuit 356. Thecombined signals applied to a conventional modulator AM modulator 358.

As shown in FIG. 26, on the return path, the AM signal is applied to aconventional AM demodulator 360, the output of which is filtered byfilters 362 and 364 for application to respective mixers 366 and 368,where the bands are restored to their desired carrier frequency.

Thus, as described above, the alternate embodiment illustrated generallyin FIG. 17 provides that microcell or PCN traffic may be carried overthe installed fiber distribution system of an existing cable TV system.In addition, the system illustrates the provision of POTS/data serviceutilizing the same system. However, the additional provision ofPOTS/data service is in no way essential for the invention.

Alternate Digital Modulation/Demodulation

In the above-described system, the digitized RF signal is converted toan analog form prior to being transported to the remote optical nodeunit 342. According to the alternate exemplary embodiment now to bedescribed, the digitized form of the RF signal may be maintained throughto the remote optical node units 342 by use of digital modulation suchas QAM modulation. In the alternate exemplary embodiment of FIG. 27A,groups 453 of transmitters apply a combined output to the input ofcorresponding analog-to-digital converters 456 (includingdown-conversion to an intermediate or baseband frequency), and theframer/multiplexer 458 frames the digitized transmitter group signals sothat these groups may be extracted from the framing structure at theother end of the link. Similarly, a demultiplexer 459 demultiplexes areceived signal and applies a corresponding digitized signal to each ofanalog-to-digital converters 457 (including up-conversion) forapplication to respective receiver groups 455. Diversity output is alsooptionally provided. The transmitter groups may, for example, contain upto ten transmitters, so that the combined digitized bandwidth isapproximately 1 MHz, consisting of approximately 300 KHz of spectrumdigitized at a 2+X rate, plus framing and control bits. The alternatedigital modulation embodiment of the head end unit 332' is illustratedin FIG. 27B. Although this embodiment shall be described with respect toQAM modulation, it shall be understood that other forms of digitalmodulation are also within the scope of the present invention. Accordingto this embodiment, the digitized RF received at the head end unit 332'is demultiplexed in demultiplexer 450, output group by group, andapplied in digital form to a plurality of QAM modulators/demodulators338', as illustrated in FIG. 28. The return traffic from a QAMdemodulator 464 is applied to a framer/multiplexer unit 452, which inturn applies the digital signal back to fiber 331B through digitallymodulated laser 174.

As illustrated in FIG. 28, QAM modulator 460 receives a digitalPOTS/data input signal, a digital video signal and a digitalPCN/microcell traffic signal. QAM modulator 460 multiplexes the inputsignals and produces a QAM modulated output signal for application to AMoptical transmitter 462, which is in turn applied to fiber 340A. On areturn path, AM optical receiver 466 receives a QAM modulated signalfrom optical fiber 340B and applies an input to QAM demodulator 464.Demodulator 464 demultiplexes the received signal and in turn produces adigital microcell/PCN signal and a digital POTS/data signal.

Referring now to FIG. 29 there is shown the alternate QAM embodiment ofmicrocell optical node 342'. Alternate optical node 342' includes an AMoptical receiver 500 receiving its input from fiber 340A. QAMdemodulator 502 receives an output from AM optical receiver 500 anddemultiplexes and demodulates the signal for application todigital-to-analog converter 504. Converter 504 outputs an intermediateor baseband frequency signal which is up-converted with mixer 506 andlocal oscillator 508 to the transmission frequency. The signal isapplied to amplifier 510, filtered with filter 512, passed throughduplexer 514 and transmitted from the main antenna unit 516. On thereturn path, the RF signal is received at the main antenna unit 516,passed through duplexer 514, filtered at filter 518 and applied to adown-converter, comprising mixer 528 and local oscillator 530. Adiversity antenna 520 is optionally provided together with filter 522and a down-converter comprising a local oscillator 526 and mixer 524.The main antenna signal and diversity antenna signals are combined usingcombining circuit 532 and applied to analog-to-digital converter 534.The output of analog-to-digital converter 534 is applied to QAMmodulator 536, which applies its output to AM optical transmitter 538,which in turn applies its output to fiber 340B for transmission to thehead end.

Thus, as described above, this alternate exemplary embodiment provides asystem for maintaining the RF or PCN signal in digital format all theway to the optical node unit 342. It thus can advantageously provide ahigher quality signal than might otherwise be obtained with AMmodulation schemes.

Transmission of Digitized RF Over Switched Telephone Network

Yet another alternate exemplary embodiment of the present invention isshown in FIG. 30. In FIG. 30, base station units 600 are connected tothe remote antenna units 602 through a switched telephone network 120,as illustrated in more detail in FIGS. 31A, 32A, 33A and 34. Asillustrated in FIG. 31A, each base station unit 600 includes radiocontroller and T1 interface circuitry 22 receiving a plurality of PCN ormicrocell channels from the MTSO. The output of each of a plurality ofgroups 610 of transmitters 23 are combined at combining circuit 612 forapplication to an analog-to-digital and digital-to-analogconverter/framing/demultiplexing circuits 614. Groups 620 of receivers28 receive an analog signal output from circuits 614. Each of circuits614 also produces an analog diversity signal, which is applied to agroup 622 of diversity receivers 28'.

Each of circuits 614 functions to convert the analog RF (after suitabledown-conversion) to a digital signal which is framed and applied to theswitched telephone network. In addition, each of circuits 614 receives asignal from the switched network, which it demultiplexes and convertsback to a corresponding analog RF signal, for application to arespective receiver group 620 or diversity receiver 622.

In the exemplary embodiment illustrated herein, it is contemplated thatapproximately ten 30 KHz, PCN or AMPS cellular channels (given current 7channel spacing requirements) may be digitized into a respective 1.05 or1.25 MHz bandwidth which may be formatted as a 44.736 Mb/s DS-3 or OC-1signal for application to the switched telephone network through a T1line or optical fiber link, with bits available for control and errordetection. AMPS, or Advanced Mobile Phone Service, is the original andstandard format for cellular service consisting of frequency modulated(FM) channels at 30 KHz spacings. However, the system could carry 15 to18 time division multiple access (TDMA) signals, or a combination ofAMPS and TDMA signals could be carried. As is well known to those ofskill in the art, TDMA is an alternative modulation technique forcellular which replaces each AMPS channel with three time-multiplexeddigital signals. Hence 5 to 6 AMPS channels are 15 to 18 TDMA channels.

Referring now to FIG. 32A, there is illustrated in more detail circuit614. Circuit 614 is essentially identical to circuit 130, as illustratedwith regard to FIG. 4, but includes a network interface circuit 630 and632 in place of digitally modulated laser 136 and digital opticalreceiver 140, respectively. Interface circuits 630 and 632 provide thenecessary T1 interface or interface to an optical path.

The remote antenna units 602 are illustrated in more detail in FIG. 33A.Antenna units 602 are essentially identical in construction to theremote antenna units 102 as illustrated with respect to FIG. 8. However,in place of the digital optical receiver 160 and digitally modulatedlaser 174, there are provided network interfaces 640 and 642 forinterfacing to the switched network 120.

The same framing structure illustrated above with respect to FIGS.6,7,22 and 23 are applicable to this exemplary embodiment of theinvention, except at lower speeds as necessary. In the case where thediversity function is provided, the return path would include additionalDS-3 or OC-1 signals, requiring additional T1 or SONET line capacity onthe return path.

Referring now to FIGS. 31B, 32B and 33B, there is shown an alternateexemplary embodiment 600' of the base station 600 illustrated in FIG.30. Alternate embodiment 600' provides that all the transmitters 23 inthe base station are applied to unit 614', which is illustrated in FIG.32B. Similarly, unit 614' services all of the receivers 28 and 28' inthe base station. Thus, the embodiment of FIG. 31B differs from theembodiment of FIG. 31A in that a single unit 614' is provided for thebase station, and the transmitters and receivers are ungrouped.

Referring now to FIG. 32B, there is illustrated analog-to digital anddigital-to-analog converter/framing/demultiplexing unit 614'. Unit 614'receives the combined input from all the transmitters 23 in the basestation 600'. A broadband digitizer 132 digitizes the combined signal.The output of broadband digitizer 132 is applied to the plurality ofdigital filters 802. Digital filters 802 each preferably include adecimation filter and a finite impulse response (FIR) filter. Decimationfilter 802A receives the 30.72 MSamples/s (12-bit wide) data stream andproduces a digitized data stream corresponding to the output of one ofthe transmitters 23 (i.e. one of the channels) consisting of anapproximately 80 KSamples/s data stream, with 12-bit samples. The 80KSamples/s rate corresponds to a sampling rate of 2.4×, of a 30 KHzsignal (the channel width). However, any rate of at least 2× satisfyingthe Nyquist criterion can be used.

Decimation filter 802 is preferably, for example, a decimating digitalfilter, Part Number HSP 43220, available from Harris Semiconductor, Inc.of Melbourne Fla. Another vendor of such filters may be ESL, a divisionof TRW, Inc. Referring back to FIG. 32B, each digital filter 802 isprogrammed to filter out of the broadband signal from digitizer 132 achannel corresponding to one of transmitters 23. Accordingly, a basestation installation with twenty transmitters would require twentydigital filters 802, to extract the digitized data stream correspondingto each transmitter. Broadband digitizer 132 digitizes the entiremicrocell traffic spectrum, which is, in the case of the original AMPSsystem, 12.5 MHz wide. In the case of twenty channels, the bandwidth tobe transported can thus be greatly reduced to 600 KHz, from 12.5 MHz.Thus, digital filters 802 greatly reduce the amount of data to betransmitted over the switched network.

A frame generator/multiplexer of generally the same design as generatormultiplexer 134', is provided to multiplex the data stream from eachdigital filter 802 onto one or more T1, SONET or other carriers. Forinstance, a single channel of 72 KSamples/s, with 12-bit samples,constitutes an 864 Kb/s serial data stream. Adding framing and controlbits, as, for example, illustrate in FIGS. 22 or 23 (with, for example,a 1-bit CRC channel, a 1-bit alarm-control/under wire channel and aframe word of 6 bits) produces a serial data stream of approximately1.54 mb/s (20 bits×72 KHz).

Frame generator/multiplexer 134' can thus multiplex the output of one ofdigital filters 802 into a DS1 format on a T1 carrier with a capacity of1.55 mb/s, or can combine multiple outputs of digital filters 802 on a44.736 Mb/s DS-3 or OC-1 signal for application of the switchedtelephone network.

A filter control circuit 803 is also provided in unit 614', and has aninput to each of digital filters 802. Filter control 803 allows digitalfilters 802 to be programmed, so that their filtering characteristics(and channel selection) may be selectively changed, if desired. Filtercontrol 803 further includes an input from radio controller 22, whichmay provide control input, in order to specify the channels to beextracted from the data stream. A network interface circuit 630',interfaces frame generator/multiplexer 134' to the switched telephonenetwork.

Referring now to FIG. 33B, there is shown alternate embodiment 602',which operates in conjunction with alternate embodiment 614'. A networkinterface circuit 640' receives one or more T1, SONET or other carriersfrom the switched telephone network, carrying digitized microcelltraffic produced by frame generator/multiplexer 134'. The digitized datastream(s) are applied to demultiplexer 162', of generally the samedesign as demultiplexer 162, which extracts the digitized stream fromeach carrier, channel by channel, and applies each individual extractedchannel to a separate one of digital-to-analog converters 164'. Theoutputs of converters 164' are combined, and applied to power amplifier24, to be broadcast through antenna 26. Each of digital-to-analogconverters 164' may be of the same design as digital-to-analog converter164 of FIG. 21B. However, unlike digital-to-analog converters 164,digital-to-analog converters 164' need only handle a single channel, andthus may possibly be of less exacting design.

Alternate embodiment of remote unit 602' further includes a plurality ofdigital filters 802, which operate in the same manner as digital filters802 of base station unit 614' to extract selected microcell digitizedchannels from the output of the broadband digitized signal from theoutput of broadband analog-to-digital converter 170. Framer/multiplexer172', of generally the same design as multiplexer 172, operates in amanner similar to frame generator/multiplexer 134' to multiplex theextracted channels onto one or more T1, SONET or other carriers, appliedto the switched telephone network through network interface 642'.

Referring again to FIG. 32B, demultiplexer 142', of generally the samedesign as demultiplexer 142, receives the multiplexed signals fromremote unit 602' through network interface 632'. Unit 142' demultiplexeseach of the channels and applies a single channel to each ofdigital-to-analog converters 144', which may be of similar design todigital-to-analog converters 164'. The output of digital analogconverters 144' may be applied to receivers 28.

As illustrated in FIG. 33B, remote unit 602' may also include adiversity path with digital filters 805 provided to extract thediversity channels from the digitized diversity signal. The extractedchannels may be multiplexed through framer multiplexer 172' onto theswitched telephone network. In base station unit 614', a diversity pathis provided from demultiplexer 142', whereby extracted diversitychannels may be applied to diversity receivers 28'. Thus, as describedabove, the alternative embodiment illustrated in FIGS. 31B, 32B and 33Bprovides digital filters to extract selected microcell channels from thebroadband digitized signal travelling to and from the remote units 602'.The extraction of selected channels provides that a much more limitedbandwidth capacity is required to carry the signals from transmitters 23to the remote units and return the received channels from the remoteunits to the base station.

In yet another alternate embodiment, the system of FIG. 11 is modifiedto transport the digitized signals over the switched telephone network,as for example illustrated herein above.

Network Interface to Cable System

FIG. 34 illustrates yet another alternate exemplary embodiment of thepresent invention, wherein the transmission of digitized RF over theswitched telephone network is combined with the transmission of the RFsignal over the cable system. More specifically, as shown in FIG. 34, anetwork interface 702 is provided at the head end unit to receivedigital PCN/microcell traffic off the switched telephone network. Thattraffic is applied to QAM modulator 460 and AM optical transmitter 462(see FIG. 28). Similarly, network interface circuitry 704 provides forapplication of digital PCN or microcell traffic to the switched network,as it is received from QAM demodulator 464. Thus, signals originatingfrom a base station 600 can be carried through the switched network tothe cable system and back again.

Thus, as described above, this alternate exemplary embodiment of theinvention provides that PCN or microcell traffic may be convenientlycarried over a switched telephone network. This operation has obviousadvantages, permitting rapid installation of additional capacity, ratherthan the necessity of installing additional transmission lines.

Thus, these alternate exemplary embodiments provide for an ability totransmit radio frequency microcell or PCN traffic through a switchednetwork and through a cable system installation.

Various modifications and alternate configurations of the embodiments ofFIGS. 17 through 34 are contemplated. An all digital configuration(similar to the embodiment of FIG. 10) of the embodiment of FIG. 17 orFIG. 27A eliminates the transmitters, receivers and analog-to-digitaland reverse conversion in the base station 330. An all digitalconfiguration for the embodiment of FIG. 30 eliminates these analogcomponents from the base stations 600. The method of installing andupgrading from the analog embodiments to the all digital embodiments canbe carried out substantially as described above with regard to theembodiments of FIGS. 2-15. Other modifications to the embodiments ofFIGS. 17 through 35 include wave division multiplexing so that the fiberpairs may be replace with a single fiber.

Passive Handoff System

Referring now to FIG. 35A there is illustrated an exemplary embodimentof a passive handoff microcell telecommunications system 800. The systemshown in FIG. 35A is of like construction to that of FIG. 2 with theexception of base station units 114', which are constructed as shown inFIG. 35B to provide passive handoff switching.

For the purposes of describing system 800, microcell areas 100 arereferred to as "microcell zones," which zones are labeled for thepurposes of one exemplary embodiment as A1-A6, B1-B6 and C1-C6. Eachzone includes an independent antenna for transmitting to and receivingfrom mobile units. Zones A1-A6 collectively comprise "Cell A," zonesB1-B6 collectively comprise "cell B," and zones C1-C6 collectivelycomprise "cell C." Each cell A, B and C has a set of reusablefrequencies to be used within the cell, according to conventionalcellular system design. Passive handoff system 800 provides that atransmission frequency or channel assigned to a mobile unit in a givencell may be broadcast from the remote unit 182 in any one of microcellzones 100 under the control of a unit 114' without interaction with orcontrol from MTSO 110. A channel can thus follow a mobile telephone unitfrom one microcell zone to another within a given cell. Accordingly,multiple microcell zones may be served by the same set of channels (i.e.transmission frequencies) allowing the signal transmission power levelwithin each zone to be minimized, and thereby avoiding undesirableinterference with adjoining microcell zones or cells. The system alsoreduces the switching load on MTSO 110. However, when a mobile unittravels from one cell to another, MTSO 110 switches the unit to a newchannel (and corresponding pair of transmit and receive frequencies) inthe newly entered cell, in a conventional manner.

Referring now to FIG. 35B, there is shown in more detail a base stationunit 114' according to the present invention. Unit 114' includes a radiocontroller 22 providing an interface between the T1 lines from the MTSO110 and the base station radio equipment. Transmitters 23-1 to 23-N(where N is a positive integer) are connected to a matrix switch 802,the outputs of which are in turn connected to a plurality of combiningcircuits 804-1 to 804-X (where X is a positive integer), which are inturn connected respectively to a plurality of digitaltransmitting/receiving units 130"-1 to 130"-X. Units 130"-1 to 130"-Xare connected to the microcell areas 102 over respective transmissionpaths 104-1 to 104-X, as illustrated in FIG. 35A.

The respective outputs of transmitter/receiver digitizing units 130"carrying the analog microcell traffic, are each applied to matrix switch808. Matrix switch 808 selectively connects any input to any one ofreceivers 28-1 to 28-N through respective outputs 806-1 to 806-X, andcombining circuits 807-1 to 807-X. A controller 810 controls matrixswitch 802 and matrix switch 808 using respective control lines 812 and814. Controller 810 receives a sample of digitized microcell trafficfrom each of the digitization units 130' over sample lines 816.

As described in more detail below, controller 18 continuously processesthe digital samples received from units 130" and in response theretocontrols matrix switches 802 and 808 in order to switch each oftransmitter units 23 through to one (or more or none) of units 130" andto connect receivers 28 to one (or more or none) of units 130". Forinstance, in one exemplary switching configuration, matrix switch 802might connect all three transmitters 23-1, 23-2 and 23-N through outputs803-1 to combiner circuit 804-1, so that all three transmitterfrequencies F₁, F₂, and F_(n) are combined and applied to unit 130"-1for digitization and transport to a microcell zone. In anotherconfiguration, transmitter 23-1 and might be connected to combiner 804-Xthrough one of outputs 803-X, while transmitter 23-2 is connected tocombiner 804-2 through one of outputs 803-2, and transmitter 23-X isconnected to combiner 804-1, through one of outputs 803-1. Matrix switch802 thus allows any one of transmitters 23 to be connected to any one ofcombiners 804, in any combination.

Switch 802 thus permits a transmission frequency to "follow" a mobileunit from one microcell zone to another. For example, with reference toFIG. 35C consider a mobile unit 820 which initiates a cellular telephonecall at a time T₁ within zone A1. In the example, mobile unit 820 islocated in a car. However, it can be hand-carried or otherwisetransported from zone to zone. To set up calls and perform control, thecontrol channel(s) for a cell A, B or C is simultaneously transmitted toand received from all zones in the respective cell, as accomplished byswitches 802 and 808. Upon call set up, which is accomplished in aconventional fashion, as for example described in "Mobile CellularTelecommunications Systems, by William C. Y. Lee, MTSO 110 assignsmobile unit 820 to a currently available channel, for example thefrequencies handled by transmitter 23-1 and receiver 28-1 (assuming atransmitter/receiver pair is currently available for assignment). MTSO110 is programmed to recognize that the channels associated withtransmitters 23-1 to 23-N and receivers 28-1 to 28-N are assigned,collectively, to cell A, which in this example consists of zones A1-A6.During the initial set up the assigned transmit and receive channels canbe transmitted to received from all zones in the cell, at least until itcan be determined which zone can handle the call exclusively.

Thus, as initially set up, mobile unit 820 transmits and receives onfrequencies F₁ and F'₁, respectively. Controller 810 constantly monitorsthe signal strength of transmissions from mobile units 820 in all zonesin Cell A as received at the antenna units of the remote units 102positioned in the zones. Signal strength in each zone is detected bysampling the digitized RF microcell traffic returning from remote units102 to units 130". While mobile unit 820 is within microcell zone A1,the strength of the received signal F'₁ is likely the greatest becauseof the proximity of mobile unit 820 to the antenna unit of remote unit102-1 in zone A1. Frequency F'₁ might, however, also be received at theantenna of remote unit 102 in zone A2, or in the more distant zone A3.Control unit 810 monitors the strength of received signal. F'₁ in all ofthe digitized microcell traffic streams received from all of remoteunits 102 in the Cell A, and, according to at least one exemplaryapproach, identifies the remote unit 102 which receives the strongestsignal at frequency F'₁. Assuming for this example, that the signal F'₁received at the remote unit 102 in zone A1 is the strongest among thezones, controller 810 signals matrix switch 802 to connect transmitter23-1 to combiner 804-1, which in turn applies its output to digitizing130"-1. Unit 130"-1 in turn transmits the digitized microcell trafficstream containing the frequency F₁ to the remote unit in zone A1, whichin turn broadcasts frequency F₁ in zone A1 (along with any otherfrequencies switched into the combiner 804-1). On the return path,controller 810 causes matrix switch 808 to connect the output ofdigitizing unit 130"-1, as received on line 806-1, to receiver 28-1.Preferably, transmitter 23-1 is connected to no other digitizing units130", such that no other remote unit 102 is broadcasting at thefrequency F₁, except for unit 102-1. Similarly, it is preferable that noother digitizing units 130" are connected through matrix switch 808 toreceiver 28-1. As a result, interference between adjacent microcellzones caused by broadcasting the same frequency is avoided andinterference resulting from a receiver 28 receiving the same frequency(at different phases and varying distortions) from more than one zone isavoided.

Extending the example further, consider now that mobile unit 820 movesfrom zone A1 to microcell zone A2 at a time t₂. As mobile unit 820 movesfrom microcell zone A1 to zone A2, controller 810 continues to sampleand detect the received signal strength of transmission frequency F'₁from all the remote units 102 in cell A. Upon movement from microcellzone A1 to A2, controller 810 should detect an increasingly strongersignal at frequency F'₁ in microcell area A2, and correspondingly areduction in signal strength at that frequency in microcell area zoneA1. When certain switching criteria are met, controller 810 performs a"passive handoff," by switching transmitter 23-1 from connection tocombiner 804-1 to connection with combiner 804-2, and correspondinglyswitching receiver 28-1 to receive its input from digitizing 130"-2. Asa result, transmission at frequency F₁ ceases at remote unit 102 in zoneA1, and the signal received at that remote unit 102 is no longer appliedthrough switch 808 to receiver 28. Thus, system 800 can passively switcha channel from one zone to another within a cell to follow a mobileunit.

The following example illustrates the operation of system 800 when themobile unit moves from one cell to another. For example, if mobile unit820 moves from microcell zone A3 to zone B1 at a time t₃, controller 810again detects a corresponding reduction in signal strength received atthe remote unit 102 in zone A3. However, no corresponding increase insignal strength in another zone in cell A is detected to trigger apassive handoff. Rather, the handoff from cell A to cell B is handed byMTSO 110 as MTSO 110 senses the movement of the mobile unit 820 betweencell A and B. Prior to leaving the cell, as the signal strengthdecreases, transmission and reception may be achieved using all zones inthe cell. As the unit 820 moves into the B cell, MTSO 110 operates toassign a new channel to the mobile unit, from frequencies assigned tocell B. The base station unit 114' serving cell B then operates in thesame manner as described above to identify the initial zone to transmitand receive from, and to perform passive handoffs within cell B.Accordingly, switching between cells A, B or C is carried outindependently of the passive handoff of assigned frequencies betweenzones in a cell. Cell B could, of course, be of conventional design witha single antenna serving the entire cell.

Thus, as described above, the present invention provides a passivehandoff system, wherein a transmission frequency is assigned to a mobileunit, and that frequency tracks or follows the mobile unit from onemicrocell zone to another under the control of controller 810, andwithout intervention from or switching of transmission frequencies bythe MTSO 110. This mode of operation is particularly advantageous incertain microcell applications, wherein multiple remote units 102 arerequired to cover an area, but there is not enough traffic density in agiven zone within the area to support a conventional cell siteinstallation. For example, a narrowed depression in the terrain, such asa ravine or along a road adjacent to a river bed may require multipleantenna installations to obtain adequate signal coverage, due toblockage from nearby terrain. Another example might be in an undergroundparking garage, or even in large office buildings where larger thannormal signal attenuation results in unacceptable signal levels.Furthermore, cell sites in some cellular systems are not located closeenough together, thus resulting in poor coverage areas between thecells. Still another example is along a traffic corridor betweenpopulation centers. For these situations and others, it is advantageousto use a passive handoff system permitting an expansion of the areacovered without assigning separate frequency sets and correspondingtransmitters and receivers for each zone within the area.

Preferably, each switch 802 and 808 provides support for at least twenty(20) transmitters and twenty (20) receivers, respectively. In addition,each of switches 802 and 808 preferably permits connection of thetransmitters and receivers to up to six digitizing units 130".Accordingly, matrix switch 802 may be used, for example, to connect upto twenty (20) transmitters (where N=20), through to any one ofdigitizing units 130". Similarly, the output from digitizers 130" may beselectively connected to any one of receivers 28, such that a single oneof digitizers 130" may be connected to all of receivers 28, or all ofthe digitizing units 130" may be connected to a single one of receivers28. However, it shall be understood that switches 802 and 808 may beadapted to handle more or less than twenty (20) receivers ortransmitters, or more or less than six (6) units 114'.

Switches 802 and 808 are preferably matrix switches, wherein thecombining function is integrated into the switch at the matrix nodes, inthe form of Wilkinson combiners using nonreflective pin diodeattenuators. Such components are available from Salisbury Engineering,Inc., of Salisbury, Md. The switches are preferably of the attenuatortype, allowing linear control of rise and fall time. Switching ispreferably make before break.

Referring now to FIG. 36, there is shown a first exemplary embodiment ofunit 130". Unit 130" is of the same design and operation as unit 130,except it additionally includes a data bus 830 connected to the buscarrying demultiplexed digitized microcell traffic from demultiplexer142 to digital-to-analog converter 144. Bus 830 is applied toparallel-in parallel-out FIFO buffer 832, which has an output enablecontrolled by an enable line 834 received from a controller 810. Whenenabled, buffer 832 outputs a replica of the digitized microcell trafficon data bus 836.

Referring now to FIG. 37, there is shown an exemplary embodiment ofcontroller 810 according to the present invention. Controller 810 ofFIG. 38A is adapted for use with unit 130" shown in FIG. 36. Controller810 includes a multiplexer 884 which is connected to the buffers 832 ineach of units 130"-1 to 130"-X, through a twelve (12) bit data bus 836with one (1) clock line. Multiplexer 884 (preferably tri-state) selectsinput from one of the busses 836, and supplies it tofast-fourier-transform (FFT) processor 856. Selection is made undercontrol of microprocessor system 860, using control line 862. FFTprocessor 856 clocks in digitized microcell traffic samples consistingof 12 bit words. Digital FFT processor 856 preferably uses a RaytheonPart No. 3310, available from Raytheon, Inc.

The output of FFT processor 856 is a plurality of 16 bit words in bins,with each bin representing the strength or amplitude of a 30 KHz channel(or channel of a PCS or other service) within the digitized cellulardata stream. The output of FFT processor 856 is applied to system 860over data bus 859, using control line 861. A select circuit 886 receivesa control signal 863 from system 860, and selectively generates signalson enable lines 834. Enable lines 834 are used to selectively enable theoutputs of buffers 832, so that FFT processor 856 can be selectivelyfilled with digitized microcell traffic samples from a selected source.Microprocessor system 860 is connected to a matrix switch driver 875,which drives matrix switches 802 and 808. The operation of controller810 as shown in FIG. 37 will be described in more detail below.

Referring now to FIG. 38, there is shown a simplified flow diagram ofthe operation of programmed microprocessor system 860 and itscorresponding control over the operation of system 800. FIG. 39 isrepresentative of both the program 900 executed by microprocessor system860 and the method of system 800. Program 900 include aninitialization/configuration routine 910. System configuration providesfor the identification of the channels serviced by base station 114'.Preferably, microprocessor system 860 includes magnetic storage mediasuch as a hard drive or the equivalent for storage of the configurationinformation and other data, together with computer programs. Onceconfigured, polling and switching operation may be invoked. In this modeof operation, microprocessor system 860 first selects (routine 912) thedigitized traffic stream for a "first" zone in the cell. In theembodiment of FIG. 37, the selection is achieved using enable lines 834.A selected one of enable lines 834 is activated to enable theacquisition and output of the microcell traffic data from acorresponding one of buffers 832. The enabled buffer 832 applies areplica of the digitized microcell traffic stream from demultiplexer 142in unit 130" to multiplexer 884, which applies the digitized trafficstream to FFT processor 856.

Routine 914 provides that FFT processor 856 is activated for loading ofthe digitized microcell traffic stream under the control ofmicroprocessor system 860 using control line 861. A buffer 832 may load,for instance, 1024 samples of the digitized microcell traffic. Asmicrocell traffic data is received from a buffer 832, FFT processor 856clocks in digitized 12 bit microcell traffic samples or words. Theoutput of FFT processor 856 comprising a series of 16 bit wordsspecifying the signal strength of the respective channels carried in thedigitized microcell traffic stream.

Microprocessor system 860 preferably employs an Intel brand "486" typemicroprocessor or better running at least 33 MHz. At this speed, thetime between selection of the digitized microcell traffic stream and thereceipt of the frequency spectrum analysis from FFT processor 856 can beless than 5 milliseconds. Once microprocessor system 860 has received(916) the frequency spectrum data from FFT circuit 856, which containsthe signal amplitude for each frequency in the zone, the data isrecorded for immediate or later analysis (routine 918). Optionally, thedate and time of the signal measurement is also recorded, together withany other parameters of interest. The polling process continues if allzones in the cell have not yet been measured within the current pollingcycle. If polling continues, the digitized microcell traffic stream forthe next zone in the cell is selected (routine 924) in theabove-described process of data acquisition analysis and storage isrepeated.

Once all zones have been measured in a current cycle, microprocessorsystem 860 determines the channel (i.e., transmitter/receiver) zoneassignments based on the signal levels recorded during the cycle. Theparticular manner in which this determination is made is not essentialto the invention, but preferably may take one of the forms describedbelow.

It is contemplated that the switching algorithms for the transmit andreceive paths of unit 114' will be different. In the transmit path, itis contemplated that the method of switching will use the coveragereceived signal strength in a given zone over a period of 1/2 second to3 seconds, with the zone with the greatest strength chosen as the activezone. Alternatively, a zone which is not currently fading, even if at alower signal strength, may be chosen. If it doesn't matter which zone isused, for example, if signal strengths are comparable, a zone may bechosen which evens out the distribution of channel assignments in thecell. Where the optimum zone cannot be determined, several or all zonescan be selected or active, for example, as might occur when a mobileunit is on the edge of a cell.

For switching receivers, instantaneous and average levels are tracked,and fades are tracked so that trends can be predicted and the switchingfrom one zone to another on the receive side can be anticipated. If thereceived signal strength is below a threshold level, then a receiver maybe connected for reception from all zones, for instance where a mobileunit is on the edge of a cell. Switching on the receive side istypically accomplished at a much faster rate of change, than on thetransmit side owing to the greater problem of reception and fading fromthe relative low power transmitters in the mobile units.

Of course, other switching algorithms for both the transmit and receivechannels are possible, and certainly those applicable to conventionalcellular switching are good candidates.

Once the new channel (transmitter/receiver) assignment has beendetermined, system 860 switches the transmitters and receivers usingswitches 802 and 808, through matrix switch drivers 875.

As an alternative to the operation specified for program 900 describedabove, channel (transmitter/receiver) zone assignments may be determinedon a continual basis after each new frequency spectrum measurement isobtained. For instance, program 900 of FIG. 38 may be modified byinserting steps 930 and 932 between step 920 and 924, and eliminatingdecision step 922. Thus, as described above, system 810 may complete ananalysis of all channels in a given zone in under 2 ms. In a handoffsystem with 6 zones, all analysis can be done in under 12 ms. Oneadvantage to such fast channel analysis is in the capability of basestation receive diversity, which may improve signal quality in areas atthe fringes of cell coverage or where signal is momentarily blocked onone zone. Since fading is a major problem in, for example, remote areas,the ability to quickly switch between receiver sources allows a form ofdiversity reception using the antennas in different zones as "diversity"antennas for each other.

A possible side advantage of fast analysis would be to accumulatestatistical data on fading that might assist service providers infinding optimum antenna/microcell placement.

As mentioned above, microprocessor system 860 may optionally record thedate and time of each measurement of the frequency spectrum of thedigitized microcell traffic stream. Accordingly, a history of channelusage and signal strength within any given channel may be readilyobtained, and later used for the purpose of reconfiguring the system,for example, by moving antenna units. Accordingly, the present inventionfurther contemplates a method of recording the use of the channelswithin the zones and the corresponding signal strength, and later usingthis information to reconfigure the system.

As an alternate exemplary embodiment, the system of FIG. 35A (and FIG.39 below) can be modified so that the digitized RF signal is carried tothe zones over a switched telephone network, as for example illustratedin FIG. 30, or modified to transport the digitized RF over a cablesystem, as for example illustrated herein.

In the alternate embodiment of FIG. 39A, base station 114' is modifiedto provide all digital base station 114" All digital base station 114",like the system of FIG. 11A, uses a digital synthesizer 212' and digitaldemodulator 224 to replace the analog RF radio equipment in the basestation. A T1 Interface 202 interfaces to MTSO 110, and applies adigital form of each telephone signal all control signal from the MTSOto each digital synthesizer 212'. Each synthesizer receives controlsignals from the controller 810' over line 812'. Each digitalsynthesizer 212' is responsive to controller 810' to create asynthesizer digital data stream for framing and transport to anassociated unit 106 in a zone, so that any combination of the channelsassigned to the cell can be broadcast in the cell.

On the return path, the digitized sample 816 is taken from thedemultiplexed digital data stream returning from the units 106, andsupplied to controller 810'. The digital samples are obtained from thedemultiplexer 221' in a like manner as described above with respect toFIG. 36. Controller 810' in turn uses the sample data as described abovewith respect to control 810 to control switching. Selector 880 can beused to select the received signal for any desired channel from any oneof demodulators 224, for application to T1 interface 202. Alternatively,selector/processor 880 is configured to process two or more of theincoming streams for each channel to create a reduced noise compositestream.

An alternate embodiment of the system of FIG. 39A is illustrated in FIG.39B. The system of FIG. 39B is similar to the system of FIG. 11B, inthat the digital synthesizer 212'" receives an analog telephone signalinput from radio controller 22, and operates like synthesizer 212' ofFIG. 11B. Similarly, digital demodulator 224' operates like demodulator224' of FIG. 11B, delivering an analog signal to radio controller 22.

Yet another two alternate embodiments of the system of FIG. 35 are shownin FIGS. 39C and 39D, which are modified in a manner similar to thesystems of FIGS. 11C and 11D, so that transport is over the switchednetwork and the synthesizer produces individual digitized channels forapplication to the network.

Referring now to FIG. 40, there is shown yet another exemplaryembodiment of the system of FIG. 35A, in this case modified tocommunicate through the switched telephone network by replacing units130" with a modified version 614" (modified to obtain the samples of thedigitized traffic stream) of circuit 614' of FIG. 32B. In thisembodiment, only those channels actually used in the antenna units 106are transported to the units, saving bandwidth in the same way as system614'.

Referring to FIGS. 41A-C, there is illustrated other alternate exemplaryembodiments of the passive handoff microcell system of the presentinvention. In the systems of FIG. 41A-C, redundancy is achieved byarranging the microcell units 102 such that each area of the cell, or"macro cell", is covered by at least two microcell units. Accordingly,in the event of a failure of one of the units, the redundant microcells103 are available to provide coverage in the area lost due to thefailure.

In FIG. 41A, a macrocell 103 is covered by three primary microcells 102and by three secondary microcells 105. In the example shown secondarymicrocells 105 are placed in a distribution similar to that of primarymicrocells 102 but rotated 45 degrees around the center of macrocell103. It should be apparent that other distributions could be usedadvantageously to provide similar redundant coverage.

In normal operation, primary microcells 102 provide full coverage overmacrocell 103. In case of a failure in one of the microcells 102,however, the two adjacent microcells 105 can provide coverage over theregion served by the failed primary microcell 102. In anotherembodiment, primary microcells 102 provide primary coverage to firstregions of macrocell 103 and secondary coverage to second regions ofmacrocell 103 while secondary microcells 105 provide primary coverage tothe second regions of macrocell 103 and secondary coverage to the firstregions.

A second method of providing redundant coverage is illustrated in twoembodiments shown in FIGS. 41B and C, respectively. In the embodimentsshown in FIGS. 41B and 41C, each microcell 102 is provided with twopower levels. In normal operation, each microcell 102 is operated at thepower level necessary to provide microcell 102 coverage. In cases where,however, a microcell 102 fails, adjoining microcells 102 are raised to ahigher power level (shown as 102'). As can be seen in FIGS. 41B and 41C,such an increase in power level provides coverage over the failedmicrocell 102. Although the microcells 102 of FIGS. 41A and 41B areshown divided into three sectors, it should be apparent that othersectorization, or no sectorization, can be used within theabove-described redundancy scheme.

Sectorization

Sectorization will be discussed next.

According to yet another aspect of the invention, the microcell systemof the present invention may be used to replace the conventional basestation transmitter 12 in a conventional cell as for example shown inFIG. 1A. In addition, as can be seen in FIGS. 41A and 41B, eachmicrocell could be split into a number of sectors, each sector driven bya directional microcell antenna unit. According to such embodiments,shown generally in FIG. 42, a sectorized antenna unit 900 having aplurality of transmit and receive antenna pairs 902 divides a micro ormacrocell into a number of sectors. Each antenna pair 902 broadcasts andreceives using a different channel set. For example, according to onepreferred embodiment, a microcell 16 is divided into three 120-degreesectors with one antenna pair 902 assigned to each sector. Each antennapair 902 utilizes ten transmit and receive channels for its sector, witha 21 channel separation between channels within the sector. In addition,according to one exemplary embodiment, there is provided seven channelseparation between channels, between sectors.

The antenna pairs 902 in each macrocell are supported by a remote unit904 which receives digitized RF for the channels in all three sectors,and converts the digitized RF into analog RF for transmission into thesectors covered by the antenna pairs 902. Remote units 904 furtherinclude analog-to-digital converters for digitizing RF received in eachsector, and for transmitting the digitized RF to the sectorized basestation units 906. Each of the sectorized base station units 906 isconnected to the MTSO 17, which in turn is connected in turn to theswitched telephone network 15.

Each sectorized base station unit 906 includes radio frequencytransmitters and receivers for each of the channel sets used in each ofthe sectors of the macrocell, and digital-to-analog andanalog-to-digital conversion units for transmitting digitized RF to theremote units and for receiving digitized RF and applying it to thereceiver units. Sectorized base station units 906 are preferablyconnected to remote units 904 over a single fiber optic link 905 usingwave division multiplexing as described above, although separatetransmit and receive links could be used if desired.

Referring now to FIG. 43, there is shown in more detail a sectorizedbase station unit 906. Each sectorized base station unit 906 includes aradio controller 22 for each of the sectors serviced by the base stationunit 906. Each of the radio controllers 22 are connected to the MTSO 17.A corresponding number of transmitter and receiver banks 912 areprovided, each with a plurality of transmitters and receivers.Preferably, according to the exemplary embodiment shown herein, eachbank 912 includes ten transmitters and ten receivers. The output of thetransmitters of each bank 912 is combined and applied toanalog-to-digital conversion unit 914, which may be of a design similarto those described hereinabove, for example as shown in FIG. 4.Analog-to-digital conversion unit 914 digitizes and frames the digitizedRF, and applies an optical output to wave division multiplexer 916,which is in turn connected to fiber 905. At the same time, opticalinformation received from remote unit 904 is applied through wavedivision multiplexer 916 to an optical filter 918 which filters out thesignal received from remote units 904 as distinct optical wavelengths,for example in the case of a three sector system, wavelengths of 1520,1550, and 1580 nm can be used. Each of the filtered, separatewavelengths is applied to the input of one or more digital-to-analogconversion units 920, which demultiplex and convert fromdigital-to-analog form RF signals received from the remote units 904,for each of the sectors serviced by the remote unit 904. The analogoutput of digital-to-analog conversion units 920 is applied to therespective receivers in each bank 912.

Referring now to FIG. 44, there is shown in more detail a remote unit904. Each remote unit 904 includes a wave division multiplexer 930connected to fiber 905. Wave division multiplexer 930 receives fromsectorized base station unit 906 the digitized optical signal carryingthe channels of all sectors serviced by remote unit 904, and applies thedigital optical signal to digital-to-analog converter unit 932, theoutput of which is an analog RF signal representative of all channelsrepresented in the sectors serviced by the remote unit 904. The analogoutput of conversion unit 932 is applied to splitter 934, which splitsthe analog RF signal into N paths (where N=the number of sectors)corresponding to channels assigned to each of the antenna pairs 902applies the analog RF to channel filter units 936. Each antenna pair 902has its own channel filter unit 936 to filter out of the RF signals fromsplitters 934, those channels to be transmitted in the respectivesector. The output of channel filter unit 936 is applied to an amplifier938, which is in turn applied to a band pass filter 940, which passesonly those channels within the band assigned to the particular sector.The output of band pass filter 940 is applied to a transmitter antenna902a of antenna pair 902. Meanwhile, a receiving antenna 902b of thatantenna pair 902, receives RF signals predominantly from within the samesector, and applies the received signals to a band pass filter 942. Bandpass filter 942 passes only those channels within the band, and appliesthe filtered radio frequency signal to analog-to-digital conversion unit944, which converts the analog RF signal to a corresponding digitizedoptical output signal at a unique optical wavelength, for example, oneof the optical wavelengths noted above. Each of analog-to-digitalconversion units 944 may be of generally the same design shown withrespect to unit 102 shown in FIG. 8. The optical outputs of each of theunits 944 is applied to optical combiner 946, which in turn applies itsoutput to wave division multiplexer 930. Digital-to-analog conversionunits 932 are preferably of generally the same design shown with respectto unit 130 in FIG. 4.

Referring now to FIG. 45, there is shown in more detail one of channelfilter units 936. Each unit 936 preferably includes a linearprogrammable pre-amplifier 950, which is used to provide the gain neededto compensate for the losses of the splitters and combiners. The outputof amplifier 950 is applied to splitter 952, which splits the analogsignal into M paths (where M=the number of transmit channels assigned toa sector) a plurality of paths. Each path in turn passes through anarrow band filter 954 tuned to the particular channel. Each narrow bandfilter 954 is preferably programmable, and designed to maintain abandwidth of 30 KHz over temperature. Preferably, this is accomplishedby first downconverting the required RF channel to a 70 MHz IF signal.The 70 MHz signal is then passed through a crystal filter in a mannerknown in the art to achieve the narrow filtering required. The IFfrequency is then upconverted to the required RF frequency. Preferably,the frequency is microprocessor controlled, and the RF frequency can beset in 1 Hz increments to the required frequency using a computer, suchas a laptop unit. Frequency stability is preferably achieved using aclock recovered from the encoded signal sent over fiber link 905.Ideally, narrow band filter 954 should be narrow enough that an adjacentchannel within the sector will be greater than 50 dB down a master clockgenerated at base station unit 906. The output of each of the narrowband filters 954 is applied to a combiner 956, which in turn providesits output to the amplifier 938. In the exemplary embodiment of thepresent invention, amplifier 938 preferably constitutes a 25 watt PA.

In the preferred embodiment, frequency offset will be minimized bysynchronizing remote unit 904 with sectorized base station unit 906. Inone such embodiment, sectorized base station unit 906 transports the RFspectrum by down-converting from RF to an IF (in, for example, the 0-30MHz range), and then digitizing. After being transported to the otherend, the IF signal is reconstructed, and then up-converted back to RF.

The down-conversion and up-conversion are implemented by mixing thesignal with a local oscillator (LO). In order for the original frequencyof the signal to be restored, the signal must be up-converted with an LOthat has exactly the same frequency as the LO that was used for downconversion. Any difference in LO frequencies will translate to anequivalent end to end frequency offset. In the embodiment describedabove, the down conversion and up conversion LO's are at locationsremote from one another. Therefore, in one preferred embodiment,frequency coherence between the local and remote LO's is established asfollows: at the host end, there is a 552.96 MHz master clock whichestablishes the bit rate over the fiber. This clock also generates a30.72 MHz clock (30.72=522.96÷18), which serves as a reference to whichthe host digitizer LO's are locked.

At the remote end, there is another 552.96 MHz clock, which is recoveredfrom the optical bit stream with the help of a phase lock loop. Becausethis clock is recovered from the bit stream generated at the host, it isfrequency coherent with the master clock. A 30.72 MHz clock is thengenerated to serve as a reference for the remote local oscillators.Because the 552.96 MHz clocks are frequency coherent, so are the 30.72MHz references, and any LO's locked to them, thus ensuring that host andremote LO's are locked in frequency.

Referring now to FIGS. 45 and 46, there is shown yet another alternateexemplary embodiment to the sectorized microcell system according to thepresent invention. In this embodiment, sectorized base station unit 906,provides that an analog-to-digital multiplexer and digital-to-analogdemultiplexer unit 960 receives a separate input from each of thechannel banks 912, and separately converts each of the RF compositesignals from the channel banks to a corresponding digitized RF stream.This digitized RF stream is in turn multiplexed into a single digitizedstream, which is output in optical form for application to wave divisionmultiplexer 916. In the reverse direction, a single digitized RF streamis received from wave division multiplexer 916, and demultiplexed into Nseparate digital streams, each corresponding to one of N sectors (whereN=3 in the example shown in FIG. 42). Each of the digital streamsrepresents a desynchronized of the analog RF received by the respectivesector antenna in pair 902. The demultiplexed digital stream is thenconverted from digital-to-analog form, and applied to each of therespective receivers in the channel banks 912.

FIG. 47 illustrates an alternate embodiment of remote unit 904 of FIG.42. Remote units 900 of FIG. 47 include a multiplexer/demultiplexer unit970, which receives the digitized stream from wave division multiplexer930, and converts the multiplexed digitized signals from each of therespective banks in the sectorized base station unit 906 shown in FIG.46. The demultiplexed data streams for each of the banks is applied torespective digital-to-analog and analog-to-digital conversion units 972which convert the digitized signal to a corresponding analog RF signal.The analog RF signal is applied to an amplifier 938, which is in turnapplied to band pass filter 940 and to transmitter antenna 902a, in amanner similar to that described for FIG. 44. Similarly, RF receivingantenna 902b is applied to band pass filter 942, which in turn appliesits output to unit 972, wherein the analog signal is converted to adigital form for application to multiplexer/demultiplexer 970. Thedigitized data streams from each of units 972 is multiplexed in unit970, converted to an optical output, and applied to wave divisionalmultiplexer 932, for transmission over fiber 905 to sectorized basestation unit 906 of FIG. 46. The digitized data stream is received bywave division multiplexer 916, in sectorized base station unit 906 ofFIG. 46, applied to unit 960. Unit 960 demultiplexes the digitizedstream into a digital stream associated with each sector and convertseach sector digital stream to a sector RF signal. The sector RF signalis applied to the receivers of the respective channel banks for thesectors.

Thus, the sectorized microcell system of the present invention allowsfor the replacement of the conventional cell site base station in aconvention macrocell. In the above described embodiments, the antennasused for each sector are directional, and are all located in the sameplace. Each directional antenna, one transmit and receive for eachsector, is then directed outwardly across the sector serviced by them.For instance, the sectors may be pie-shaped, with the directionalantennas positioned at the center of the pie. Alternatively,nondirectional antennas could be used and positioned at differentlocations in the cell site. In such a case, the antennas are coupled tothe cell site through coaxial cables. In addition, though the abovesectorization examples have been described using antenna pairs, itshould be obvious to one skilled in the art that sector units having oneantenna, or even units having three or more antennae may be usedadvantageously within such a system. Furthermore, although the examplesdescribed entail only the digitization of RF signals generated from thetelephone signal received from the MTSO, it should be apparent that thetechniques of digital synthesis described in the context of FIG. 10 etal. also apply to a sectorized microcell system. Diversity channels mayalso be implemented as described above.

Finally, although each of the examples above describes the use of ananalog RF signal transmitted and received by each remote unit, it shouldbe obvious that the above system and method can be appliedadvantageously to a digital RF cellular system in a manner well known inthe art.

Thus, as described above, the sectorized cell replacement systemprovides for greater reuse of channels, by dividing conventional cellsor even microcells into a plurality of sectors. Furthermore, the systemprovides all the benefits and advantages of the microcell systemsdescribed hereinabove, wherein the transmitters and receivers for allthe channels in the cell are centrally located in a convenient andinexpensive location.

Thus, as described above, the present inventions provide a variety ofdigital systems and methods for transporting cellular traffic to andfrom antenna units, and for passively switching. Although theinvention(s) has been described in its preferred form, those of skill inthe art will recognize that many modifications and changes may be madethereto without departing from the spirit and the scope of the claimsappended hereto.

We claim:
 1. A method of passive switching in a cellular phone systemhaving a plurality of macrocells including a first macrocell, eachmacrocell sharing a common set of channels, comprising the steps of:(a)dividing the first macrocell into a plurality of primary microcells; (b)placing a plurality of primary microcell antenna units so as to providecoverage over the first macrocell; (c) dividing the first macrocell intoa plurality of secondary microcells; (d) placing a plurality ofsecondary microcell antenna units to provide coverage over the firstmacrocell overlapping the coverage of the primary microcells; (e) at abase station,(1) generating a digitized representation of a telephonesignal received from a mobile telephone switching office, (2) selectinga microcell from said plurality of primary and secondary microcells, and(3) transmitting the digitized representation to the microcell antennaunit of the selected microcell; (f) at the selected microcell,(1)receiving the digitized representation, (2) generating a correspondingRF signal by digital-to-analog conversion, and (3) broadcasting the RFsignal in the selected microcell; (g) at each of the plurality ofprimary and secondary microcells,(1) receiving RF signals from mobileunits within the set of channels, (2) converting the RF signals receivedto corresponding digitized RF signal representations, and (3)transmitting the RF signals back to the base station; (h) at the basestation,(1) receiving the digitized RF signal representations from theprimary and secondary microcells, and (2) monitoring the digitized RFsignal representations from each of the primary and secondarymicrocells; and (3) based on an energy level of each channel in eachmicrocell,(A) selectively controlling the channels broadcast into eachof the primary and secondary microcells by controlling the generation ofthe digitized representations as performed in step (e), and (B)selectively choosing a particular microcell from the plurality ofprimary and secondary microcell in which a received channel is receivedso that passive switching is accomplished.
 2. The method according toclaim 1 wherein the step of generating a digitized representation of atelephone signal received from a mobile telephone switching officecomprises digitally synthesizing a digitized representation of RFsignals to be transmitted for each channel from the telephone signalreceived from the mobile telephone switching office.
 3. The methodaccording to claim 1 wherein the step of generating a digitizedrepresentation of a telephone signal received from a mobile telephoneswitching office comprises generating a digitized representation of RFsignals generated for each channel.